MARSIS observations of field-aligned irregularities and ducted radio propagation in the Martian ionosphere
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MARSIS observations of field-aligned irregularities and ducted
radio propagation in the Martian ionosphere
D. J. Andrews1 , H. J. Opgenoorth1 , T. B. Leyser1 , S. Buchert1 , N. J. T. Edberg1 , D. D.
Morgan2 , D. A. Gurnett2 , A. J. Kopf2 , K. Fallows3 , P. Withers3,4 .
Corresponding author: David J. Andrews, Swedish Institute of Space Physics
(Uppsala), Box 537, Uppsala 75121, Sweden.
arXiv:1808.05084v1 [physics.space-ph] 15 Aug 2018
david.andrews@irfu.se
1: Swedish Institute for Space Physics, Uppsala, Sweden
2: Department of Physics & Astronomy, University of Iowa, IA, USA
3: Center for Space Physics, Boston University, MA, USA
4: Department of Astronomy, Boston University, MA, USA
August 2018. Preprint accepted for publication in
J. Geophys. Res. (Space Physics).
Abstract
Knowledge of Mars’s ionosphere has been significantly advanced in recent years by observations from
Mars Express (MEX) and lately MAVEN. A topic of particular interest are the interactions between
the planet’s ionospheric plasma and its highly structured crustal magnetic fields, and how these lead
to the redistribution of plasma and affect the propagation of radio waves in the system. In this paper,
we elucidate a possible relationship between two anomalous radar signatures previously reported in
observations from the MARSIS instrument on MEX. Relatively uncommon observations of localized,
extreme increases in the ionospheric peak density in regions of radial (cusp-like) magnetic fields and
spread-echo radar signatures are shown to be coincident with ducting of the same radar pulses at higher
altitudes on the same field lines. We suggest that these two observations are both caused by a high
electric field (perpendicular to B) having distinctly different effects in two altitude regimes. At lower
altitudes, where ions are demagnetized and electrons magnetized, and recombination dominantes, a
high electric field causes irregularities, plasma turbulence, electron heating, slower recombination and
ultimately enhanced plasma densities. However, at higher altitudes, where both ions and electrons are
magnetized and atomic oxygen ions cannot recombine directly, the high electric field instead causes
frictional heating, a faster production of molecular ions by charge exchange, and so a density decrease.
The latter enables ducting of radar pulses on closed field lines, in an analogous fashion to inter-
hemispheric ducting in the Earth’s ionosphere.
Key points:
• MARSIS on MEX observed both echoes from distant ionospheric irregularities as well as locally
ducted echoes in close succession
• Both effects are consistent with the presence of extended, small-scale field-aligned density irreg-
ularities
• We suggest that these may be the direct result of strong electric fields present at ionospheric
altitudes
11 Introduction ders of magnitude smaller than the electron plasma
frequency fpe throughout the Martian ionosphere,
A diverse family of plasma instabilities are known such that radio wave propagation is not signifi-
to act in the Earth’s ionosphere, producing a range cantly affected by the magnetic field itself.
of ionospheric irregularities with many different In this paper we present data from the Mars
characteristics [see e.g. Fejer and Kelley, 1980]. Advanced Radar for Sub-Surface and Ionospheric
While some are of purely academic interest, oth- Sounding (MARSIS) instrument, the main antenna
ers, such as equatorial ‘spread-F’ can also impair of which is a 40 m tip-to-tip dipole. When operated
various radio communications. Despite the gener- in active ionospheric sounding (AIS) mode [Picardi
ally weaker magnetic field of Mars, many of the et al., 2004; Gurnett et al., 2005] discrete pulses are
same instabilities can be expected to act and mod- transmitted at stepped central frequencies between
ify the ionosphere on a range of spatial and tempo- ∼100 kHz and ∼5.5 MHz. The part of the pulse
ral scales. In this paper, we provide further anal- propagating in the nadir direction is specularly re-
ysis of recently published observations of Martian flected from the Martian ionospheric plasma at an
ionospheric irregularities, linking enhanced plasma altitude where the sounding frequency f = fpe . A
densities at low (below ∼200 km) altitudes with reflecting structure, such as an ionospheric layer,
ducted radio propagation at higher altitudes. forms a trace in the reflected signal when plotted
The ionosphere of Mars, formed principally as a function of sounding frequency and delay time
through the photo ionization of atmospheric CO2 , - a so-called ionogram. The elapsed time between
is structured by variations in the ionization rates, transmission of the pulse and its receipt yields a
neutral winds, solar wind interaction, and the measure of the distance to the reflection point,
structure of crustal magnetic fields at ionospheric although a full numerical inversion must be per-
altitudes (see e.g. recent reviews by Witasse et al. formed to remove the effect of the dispersion of the
[2002], Nagy et al. [2004], Withers [2009]). Obser- pulse as it propagates through the plasma [Morgan
vations made by the Mars Global Surveyor (MGS), et al., 2013]. The principal purpose of this mode of
Mars Express (MEX), and most recently the Mars operation is therefore to obtain vertical profiles of
Atmosphere and Volatile EvolutioN (MAVEN) mis- plasma density from the altitude of the spacecraft
sions have shown that the crustal fields are respon- down to the peak of the ionospheric plasma density.
sible for establishing both large-scale changes in Close to the terminator and in regions of strong
the absolute ionospheric plasma density [e.g. Nils- crustal fields, Mars’s ionosphere departs signifi-
son et al., 2011; Dubinin et al., 2012; Andrews cantly from the ideal case of horizontally stratified
et al., 2013], small-scale variability [e.g. Brain et al., medium. This can give rise to reflected signals in
2007; Gurnett et al., 2010; Andrews et al., 2015a], the MARSIS data that arrive at the radar from off-
and stable ionospheric “upwellings” [Gurnett et al., nadir directions, i.e. received at oblique incidence.
2005; Duru et al., 2006; Andrews et al., 2014]. Many such oblique echoes have been shown to be
Crustal fields also either control or at least modu- associated with reflection from stable, large-scale
late the character of Martian auroral emissions [e.g. ionospheric upwellings in regions of radial (verti-
Bertaux et al., 2005; Brain et al., 2006]. cal) crustal fields [Gurnett et al., 2005; Duru et al.,
While the combined lack of a planetary dynamo 2006]. However, as MARSIS does not discriminate
field and the presence of intense crustal fields is between the arrival directions of signals received,
unique to Mars within the solar system, the re- the inference of the source of these oblique echoes
sulting magnetic morphologies at ionospheric alti- is made in relation to models of the crustal field
tudes bear some similarities to both the equatorial at and in the vicinity of the spacecraft. Other
Earth ionosphere with near horizontal fields, and ionospheric structures can also give rise to oblique
the auroral or cusp-like ionosphere with near ver- echoes, for example horizontal variations in plasma
tical fields. In contrast to Earth, the inclination of density at the terminator [Duru et al., 2010].
Martian magnetic field can reverse on very small Comparable top- and bottom-side (ground
length scales (∼100 km), comparable to the typ- based) ionospheric radar sounders have been rou-
ical ionospheric ion gyroradius. The electron cy- tinely operated at Earth for many decades. Spread-
clotron frequency fce typically remains several or- F was found to be a common feature of Earth’s
2F-region ionosphere, in which an otherwise sharp and ∼250-1000 times the local cyclotron frequency
reflected radar signal is spread over a much larger fce . Zhang et al. [2016] argue that these ducts
apparent range, indicating reflections received from are artificially generated (as opposed to naturally
a disturbed plasma in contrast to a stably strat- occurring) field-aligned structures, formed in the
ified layer. A connection between instances of Martian ionosphere by the relatively high power
spread-F radar signatures and localized ionospheric MARSIS sounder (the radiated energy being large
plasma density depletions (sometimes referred to as compared to the typical thermal energy content of
‘plumes’) often detected nearby was suggested in the local plasma). In the case of similar artificial
several studies in the late 1970s [Kelley et al., 1976; field-aligned irregularities responsible for ducting in
Woodman and La Hoz , 1976; McClure et al., 1977]. the Earth’s ionosphere, the required density cavity
Spread-F and plumes are typically observed in the grows in the field-aligned direction in response to
hours after local sunset, triggered by a strength- the sounding pulses when the ratio fpe /fce is close
ening eastward electric field just around sunset, to a (low) integer value [Benson, 1997].
the pre-reversal enhancement [e.g. Woodman and In this paper, we further examine one of the
La Hoz , 1976]. These under-dense plumes, formed events first published by Zhang et al. [2016], that
below the ionospheric peak in regions of near- of 2005 day 318 (MEX orbit 2359), being the most
horizontal magnetic field, grow and propagate ver- well-formed example of ionospheric ducting produc-
tically through the Raleigh-Taylor instability, as ing a so-called ‘epsilon’ signature in several iono-
first suggested by Dungey [1956]. As the plumes grams. We investigate possible causal connections
rise to higher altitudes they perturb the surround- to another uncommon process in the Martian iono-
ing ionosphere, forming the irregularities that are sphere – the apparent extreme localized heating of
responsible for the observed radar spread-F. The the ionospheric electron plasma at altitudes close
resulting density depletions extend along the hor- to the peak density. This related process was first
izontal fields, and can reach a stable situation in observed at Mars by Nielsen et al. [2007] in data
the topside ionosphere. Space-borne radar experi- from the same orbit, among others.
ments were then found to occasionally pass through
these plumes, and observe the ducted propagation
of radar signals along the field-aligned density cavi- 2 Observations
ties [e.g., Muldrew , 1969; Dyson and Benson, 1978;
In Figure 1 we show four ionograms obtained on
Platt and Dyson, 1989]. These ducts can extend
orbit 2359, over a period of ∼5 minutes close to pe-
over large distances, often leading to the reception
riapsis at similar altitudes and longitudes, though
of reflections from the conjugate hemisphere of the
with significant variation in both latitude and so-
planet, in addition to the typical reflections from
lar zenith angle. The format of each is identical,
nadir incidence. Propagation of sounding pulses
with reflected intensity shown color-coded versus
within these ducts leads to rather unusual traces in
the sounding frequency f and delay time. Posi-
an ionogram, with more complex appearance than
tional information for MEX is shown on the up-
the reflection from a nominal ionosphere. Radar
per edge of each ionogram. All four were taken on
spread-F is also routinely associated with the pres-
the dayside, and each shows a clear ionospheric re-
ence of ducted echoes [Muldrew and Hagg, 1969].
flection at a delay of ∼1-1.5 ms. Each individual
Ducted propagation of radio waves was first iden-
ionogram is acquired over an interval of ∼1.5 s.
tified in the Martian ionosphere by Zhang et al.
Figure 1a shows a fairly typical ionospheric re-
[2016]. In a survey of ∼8 years of data from MAR-
flection trace visible from ∼1.4 to ∼3.2 MHz, with
SIS, seven ducted propagation events were found
the characteristic curve towards larger delays near
by manual inspection of the data, each of which
the highest frequencies, indicating propagation of
occurred while the spacecraft was located on the
sounding pulses down to the ionospheric peak den-
dayside (SZA < 63◦ ) and close to its periapsis (al-
sity (the so-called “critical frequency”). Numeri-
titude h < 400 km). Sounding pulses with fre-
cal inversion of this trace using the technique de-
quencies f greater than ∼1.1 MHz were found to
scribed by Morgan et al. [2013] yields a peak alti-
be ducted, corresponding to ∼3 times the local
tude of approximately 130 km, in line with expec-
plasma frequency fpe at the spacecraft in each case,
32359: 2005-11-14T06:12:49 2359: 2005-11-14T06:14:04
Lat: -66 , Lon: 193 Alt: 284 km, SZA: 53 Lat: -61 , Lon: 192 Alt: 269 km, SZA: 48
0
(a) (b)
1 12
1
log10 V2 m 2 Hz
2
Delay / ms
3 14
4
16
f / MHz: 0
5
1 2 3 4 5 0 1 2 3 4 5
ne/104cm 3 : 0.0 1.2 5.0 11.2 19.8 31.0 0.0 1.2 5.0 11.2 19.8 31.0
2359: 2005-11-14T06:16:50 2359: 2005-11-14T06:17:28
Lat: -50 , Lon: 191 Alt: 271 km, SZA: 37 Lat: -47 , Lon: 190 Alt: 279 km, SZA: 35
0
(c) (d)
1
2
Delay / ms
3 T2
T1+T2
4
2T1+T2
f / MHz: 0
5
1 2 3 4 5 0 1 2 3 4 5
ne/104cm 3 : 0.0 1.2 5.0 11.2 19.8 31.0 0.0 1.2 5.0 11.2 19.8 31.0
Figure 1: Four MARSIS ionograms obtained on orbit 2359. The received signal on the antenna is color-
coded versus both delay time τ and sounding frequency f . Equivalent plasma densities are marked on the
lower axis, and spacecraft position (latitude, longitude, altitude and solar zenith angle) in planetographic
coordinates above the upper edge. White triangles along the upper edge indicate f = fpe and f = 2fpe .
4tations based on statistical descriptions of the Mar- commonality is apparent with the ‘spread-F’ sig-
tian ionosphere at this location [e.g. Morgan et al., natures seen often in Earth’s ionosphere, as large-
2008]. The local plasma resonance at ∼0.7 MHz scale (compared to the sounding wavelength) den-
is clearly visible as a intense vertical line in the sity structures scatter the reflected rays along mul-
ionogram, and a second harmonic is also present. tiple paths.
From this we infer that the spacecraft is embedded Nielsen et al. [2007], in an analysis of the ion-
in a plasma of density ne ≈ 6500 cm−3 . Similarly, gram shown in Figure 1b and other similar features
several evenly spaced horizontal lines are present observed on the same orbit, suggested that these
at low frequencies at multiples of the local elec- greatly enhanced peak densities could be caused by
tron gyroperiod, indicating a local magnetic field a two-stream (Farley-Buneman) instability acting
of ∼100 nT. No surface reflection is visible in this in the ionosphere. Ionospheric plasma flows, driven
or any of the following ionograms, likely due to the ultimately by the action of the solar wind, would
presence of a highly collisional low-altitude plasma set up such two-stream interactions when the mag-
layer somewhere below the main peak, in which netized electrons are deflected upon flowing into
even high-frequency waves are collisionally damped a region of intense, irregular crustal fields, while
to the point that no reflected rays are able to be the heavier ions are less perturbed. Ultimately,
received. this process is suggested to locally heat the elec-
The ionogram shown in Figure 1b was obtained tron plasma and therefore reduce the recombina-
at slightly lower altitudes and marginally closer to tion rate, establishing a higher overall plasma den-
the sub-solar point than that shown in Figure 1a. sity, and could furthermore provide a source for the
The local magnetic field has increased significantly density fluctuations evidenced by the delay-spread
to >170 nT, as the spacecraft moved into a region of reflections.
more intense crustal magnetic fields, while the local Figure 1c was obtained shortly after a second
plasma resonance frequency dropped to ∼0.5 MHz, such peak density enhancement and delay spread-
(ne ≈ 3100 cm−3 ). Measurements of magnetic field ing event. The nadir trace from the ionosphere im-
strength by MARSIS are limited by the characteris- mediately below the spacecraft has now returned
tics of the instrument, such that fields weaker than to a much more sharp reflection, without any ap-
∼5 nT or stronger than ∼190 nT cannot be re- preciable degree of delay-spreading. Meanwhile,
solved due to aliasing. The ionospheric reflection the local plasma density has also risen, and is now
is markedly different than that observed in Fig- broadly consistent with that in Figure 1a. However,
ure 1a, just northward of the location of the space- a further trace is also discernible below the princi-
craft in Figure 1b. Firstly, the maximum frequency pal nadir reflection, extending still to high frequen-
of the ionospheric trace has significantly increased cies. We suggest that this additional trace is asso-
to ∼4.5 MHz (ne ≈ 2.4 × 105 cm−3 ). This indi- ciated with the peak density enhancement, having
cates a peak plasma density in the ionosphere well effectively “detached” from the original feature and
above the ∼1.3×105 cm−3 expected at this solar moved to higher delays in the intervening sound-
zenith angle according to models of the ‘nominal’ ings, appearing now in this ionogram at delays of
ionosphere [e.g. Morgan et al., 2008]. Furthermore, ∼1.9-2.4 ms. This is consistent with the source
the lack of any significant curvature in the trace of the reflection being stationary in the ionosphere
at the highest frequencies prevents us from making as the spacecraft recedes from it. The enhanced
safe conclusions about the maximum plasma den- density trace is in this case patchy, still does not
sity in this location, and the true value could well be display a clear curvature at its highest frequencies,
higher even than given. Here, the ionospheric trace and the maximum frequency of this trace contin-
is everywhere more spread out in delay than the ues to extend well above the peak frequency of the
sharp nadir reflection shown in Figure 1a, indicat- nadir trace.
ing a very non-planar ionosphere. This spreading This configuration of two distinct traces in a sin-
takes place over ∼0.5 ms in delay, corresponding gle ionogram bears some similarity to the so-called
to ∼75 km in apparent range, although the typi- ‘oblique echoes’ associated with ionospheric up-
cal scales of the associated irregularities could well wellings, as studied e.g. by Duru et al. [2006]. This
be much smaller owing to dispersive effects. Some enhanced density trace does indeed arrive at the
5spacecraft at oblique incidence, and its variation versed the paths T1, T2 and T1 again, before being
along the orbit is also necessarily similar to the hy- received on the antenna.
perbolic traces, as is the case for any reflection from Instances of the unusual ionospheric signatures
a target fixed in the ionosphere. However, its sig- shown in Figure 1b and d are apparently rare in
nificantly elevated frequency seems to reliably rule the MARSIS data set. Zhang et al. [2016] report a
it out as simply due to an otherwise unremarkable total of 7 observations of ducted wave propagation
ionospheric upwelling of the type extensively stud- in an analysis of ∼8 years of data, with the ex-
ied previously. Furthermore, Andrews et al. [2014] ample shown here in Figure 1d being one of the
have demonstrated that the echoes associated with clearest such detections. Meanwhile, the signifi-
ionospheric upwellings are highly repeatable, being cantly enhanced densities and irregularities associ-
detectable in all orbits that cross over the same re- ated with the Farley-Buneman instability were ob-
gion. Their relative stability and repeatability is in served by Nielsen et al. [2007] on a total of three
contrast to these peak density enhancements, which orbits from the first year of MARSIS operations.
are only very rarely observed [Zhang et al., 2016]. That they both occur, in their respectively clearest
The final ionogram shown in Figure 1d depicts and most extreme examples, on the same orbit and
one of the so-called ‘epsilon’ signatures, reported within a matter of minutes of each other clearly
first in the Martian ionosphere by Zhang et al. warrants further investigation.
[2016]. In addition to a more-typical ionospheric re- In Figure 2 we show all data obtained by MAR-
flection similar to that seen in Figure 1a, three con- SIS during the relevant segment of orbit 2359. Tim-
nected traces are are also visible over much larger ings of the four individual ionograms shown in Fig-
delays of ∼3-4.4 ms. These branches of the epsilon ure 1 are indicated by the labels on the upper edge
trace are labelled ‘T2’, ‘T1+T2’ and ‘2T1+T2’ in of the figure. MARSIS data is shown the format
common with the terminology used by Zhang et al. of a ‘spectrogram’ in Figure 2a, in which the indi-
[2016]. Zhang et al. [2016] suggest that this partic- vidual ionograms are processed by integrating the
ular signature is the result of ducted propagation received signal at a given frequency over all delays.
of the MARSIS sounding pulse in a plasma cavity, The resulting plot of signal intensity as a function of
by analogy to similar signatures noted in Earth- sounding frequency is color-coded at plotted versus
orbiting topside ionospheric sounders [e.g., Dyson time along the orbit. So processed, the local plasma
and Benson, 1978]. By their estimation, this sig- resonances (vertical lines in Figure 1) are visible
nature is persistent for four consecutive ionograms here as horizontal lines, with spacing changing with
(∼30 s), as the spacecraft moves through the den- time as the local plasma density changes along the
sity structure responsible for the ducting. In this orbit. Figure 2b displays a MARSIS ‘radargram’,
instance, the epsilon signature is visible at frequen- i.e. a cut through the ionograms at fixed frequency
cies from ∼1.4 to ∼2.1 MHz, i.e. from about twice f = 1.9 MHz, plotted versus apparent altitude in-
the local plasma frequency to about half of the stead of delay. Here, the intense horizontal line at
maximum plasma frequency observed at the iono- ∼100 km apparent altitude is the nadir reflection.
spheric peak. At the spacecraft, the magnetic field The lower panels c - h then show measured local
is close to horizontal, still intense and dominated by and peak electron densities, magnetic field inten-
crustal sources, and any field-aligned density cavity sity, magnetic inclination angle, the ratio fpe /fce ,
would therefore be expected to extend almost hor- and the SZA and altitude of MEX. Throughout
izontally away from the spacecraft. Ducting of the the interval shown, the spacecraft moved to pro-
wave along these closed field lines provides an op- gressively lower solar zenith angles, and periapsis
portunity for reflections to be detected from both was reached at 06:15. Vertical red dashed lines
single and multiple ‘hops’ through this cavity (i.e., and shading through each panel span the period in
pulses that have been reflected from the ends of the which the epsilon signature was reported by Zhang
cavity two or more times, returning to the space- et al. [2016], while vertical black dashed lines and
craft with sufficient intensity within the receiving grey shading show the three peak density increases
interval of the sounder, as discussed e.g. by Zhang noted by Nielsen et al. [2007] on this orbit.
et al. [2016]). In this example, the trace labelled The time variation of the peak frequency of the
‘2T1+T2’ is associated with a pulse that has tra- ionospheric reflection can be discerned by exam-
61-a 1-b 1-c 1-d
4 a 12
1
Log10V 2m 2Hz
3
f / MHz
2 14
1
16
b 12
1
0
Log10V 2m 2Hz
Alt./km
200 14
400
16
106 c
105
3
ne / cm
104
103
102
d
300
200
|B|/nT
100
90 e
45
/ deg
0
45
10903 f
h / km SZA/deg fpe/fce
102
g
60
30
0
h
400
200
06:14 06:16 06:18 06:20 06:22
2005-318
UTC HH:MM
YYYY-DOY
7
Figure 2: Caption next page.Figure 2: Timeseries plots of MARSIS data obtained on 2005-318 (14 November, orbit 2359). a) Inte-
grated reflected signal received color coded as a function of sounding frequency f and time (a so-called
‘spectrogram’). Tick marks on the upper edge indicate the timing of the ionograms displayed in Fig-
ure 1. b) Reflected signal at a constant frequency f = 1.9 MHz versus apparent altitude h (a so-called
‘radargram’). c) Local plasma density measured at the spacecraft altitude (black points, with vertical
bars indicating uncertainty), and peak ionospheric plasma density (red points) determined from the
maximum frequency of the ionospheric reflection. Correspondingly colored dotted lines give expected
nominal values at this location, to guide the eye in accounting for variations due the spacecraft motion
in altitude and SZA. d) Magnetic field magnitude measured at the spacecraft (black points) by MARSIS.
Points with a surrounding red circle are likely erroneous, aliased values. The blue line shows the expected
crustal magnetic field magnitude at the location of the spacecraft according to the model of Cain et al.
[2003], while the red line gives the magnitude of the radial component of the field (dashed negative)
from the same model. e) Magnetic zenith angle δ, with δ = 0◦ indicating horizontal fields, and δ = ±90◦
vertical fields (positive upward). f) Black points show the ratio of the plasma and cyclotron frequencies
fpe /fce at the spacecraft, computed from the black points in panels c and d. The blue trace instead
uses the modelled magnetic field magnitude in the calculation. g, h) spacecraft solar zenith angle and
planetographic altitude h. Intervals shaded grey in the lower panels, bounded by black dashed lines
indicate regions of peak ionospheric plasma density increases, as studied first by Nielsen et al. [2007].
The red shaded and hatched region indicates the interval for which the epsilon feature was observed.
ining the highest frequency signals in Figure 2a the effective scale height of the topside ionosphere
which rise above the dark blue background in- above this.
tensity. Within the three grey shaded intervals, Within the presentation format of Figure 2a, the
the peak frequency significantly increases, indicat- epsilon signature seen in Figure 1d is not visible
ing increases in peak plasma density (panel c, red as the intensity of its three branches is compa-
points), along with the delay spreading leading to rable to or weaker than the nadir ionospheric re-
a broader signature at the same time in panel b. flection, and does not therefore contribute signifi-
Each of these peak density increases is observed cantly to the log-scaled intensities shown. However,
in regions of near-radial crustal fields, evidenced in panel b, the cut through the received signal at
by the dominant contribution of the radial field to f = 1.9 MHz shows the epsilon traces as a cluster
the total modelled value (panels d and e). The of bright ‘pixels’ at the point marked by the white
peak density increase is significant compared to character. As was reported by Zhang et al. [2016],
the measurement accuracy, as seen from the gen- the branches of the epsilon signature converge with
erally close agreement between the measured and time, and the whole structure apparently recedes
expected peak densities (panel c, red points). The from the spacecraft and moves to higher frequen-
first and last peak density increases marked on cies. Only the upper ‘T2’ branch of the epsilon is
the figure are the most significant in both ampli- clearly persistent in Figure 2b, and we have placed
tude and duration, while the central event start- a white diagonal bar directly above it to guide the
ing just prior to 06:16 is both shorter in duration, eye. While its intensity remains weak compared
and with a less pronounced density increase. It to the nadir reflection throughout the period, we
can be clearly seen that the local plasma density nevertheless suggest that the reflections discernible
in each case decreases during these intervals, with below the white bar are attributable to the same
the black points in panel c dropping by up to a fac- single structure, extending well beyond the interval
tor of ∼5 compared to values at the approximate suggested by Zhang et al. [2016], clearly emerging
boundaries of these intervals. This fact was not re- from the nadir reflection at ∼06:16:15. We indi-
ported by Nielsen et al. [2007], and indicates a ma- cate the extension of the interval for which a trace
jor reconfiguration not just of the deep ionosphere associated with the epsilon signature is visible by
close to the peak, but also a significant reduction in the red hatched region in 2. Significantly, this leads
8to a clear connection between the epsilon signature the same manner as the high frequency echo shown
reported by Zhang et al. [2016] and the peak den- in Figure 1b indicates that the ducted pulses are
sity enhancements reported by Nielsen et al. [2007], propagating parallel to the field within a narrow
with the final reflection site of the T2 branch of density cavity. Scattering from the density irregu-
the epsilon trace (the ionospheric footprint of the larities, which produced the delay-spread echoes no
ducting field line) being co-located with the peak longer occurs when the irregularities instead duct
density enhancement. the sounding pulses along the field.
In order to investigate the configuration of the Throughout the interval shown in Figure 2, lo-
crustal magnetic field throughout this periapsis cal plasma oscillations are also produced and de-
pass, we display the magnetic zenith angle δ in tected by the sounder, visible as the most intense
Figure 2e. Purely vertical (cusp-like) fields have quasi-horizontal lines in panel a, at frequencies of
δ = ±90◦ , while horizontal fields have δ = 0◦ . ∼0.2-0.9 MHz. From these, the local plasma den-
It is readily apparent that the interval for which sity can be determined as shown by the black cir-
the epsilon signature is detected occurs in fields cles in Figure 2c. Generally these are elevated with
that are closer to horizontal than vertical, the av- respect to their long-term averages at the same lo-
erage value of δ during the red shaded interval be- cation, according to the model of Andrews et al.
ing ∼25◦ . Furthermore, we note a monotonic de- [2015b], shown by the dotted black line. Through-
crease in δ from near-vertical fields at the spacecraft out this orbit, the modelled values lie significantly
at the time when the immediately preceding peak below the data, although the along-orbit variation
density enhancement was detected, to the near- is well represented. The underestimated values
horizontal fields when the ducted echoes observed. are likely due to seasonal effects not present in
This smooth variation of δ from values of ∼70◦ to the simple model. No clear drop in local electron
∼15◦ indicates that throughout this period, ∼06:15 density is present during the interval highlighted
to ∼06:18, MEX traversed a single ‘arcade’ of mag- by Zhang et al. [2016], although it does end with
netic fields, including a single ‘cusp’, above which an abrupt increase in density for a single ionogram.
the peak density enhancement was detected. Fur- Instead, in our estimation the signatures of the ep-
ther towards the sub-solar point, but still on the silon structure extend to marginally earlier times,
same crustal field lines, the ducted sounding pulses beyond those reported by Zhang et al. [2016], to
were later observed. While the crustal field also the start of a (brief) peak density enhancement re-
does vary somewhat in the cross-track direction, ported by Nielsen et al. [2007] at ∼06:15:50. This
it does generally display a high degree of zonal does appear to coincide with the spacecraft cross-
symmetry in this region, such that the dominant ing a sharp local density gradient, and moving
variations in both direction and intensity are in through a local density cavity from ∼06:15:50 to
the north-south direction, and more closely aligned ∼06:17:50, with a relative depletion of ∼10-30%.
with the spacecraft orbital trajectory. Thus, while However, only a few % depletion is required to
MARSIS does not resolve angle of incidence di- support ducted propagation and the production of
rectly for received reflections, we are nevertheless the epsilon signature, on the basis of related obser-
confident that the most significant features present vations from Earth-orbiting topside sounders [e.g.
in the data on this orbit are likely to be reflections Muldrew , 1963].
from locations close to the trajectory. The correspondence between the observed iono-
As the spacecraft moves through the ionospheric spheric signatures and the crustal field structure
irregularities, from a region of near-vertical to near- is further depicted in Figure 3, in which the or-
horizontal crustal fields, the received signals trans- bit trajectory of MEX is projected onto the surface
form seamlessly from the high-frequency, delay- of Mars. Figure 3a and b also show the crustal
spread echo seen in Figure 1b, to the multiple field magnitude |B| and inclination angle δ, respec-
ducted traces seen in Figure 1c-d. Meanwhile, a tively, calculated again from the Cain et al. [2003]
nominal nadir reflection is uniformly visible once model at 300 km altitude (corresponding approxi-
the high-frequency echo is no longer observed. That mately to the altitude of MEX during the interval).
the individual branches of the epsilon trace seen in The peak density enhancements, highlighted by the
Figure 1d do not also undergo delay-spreading in three white segments are seen to occur in regions
9Latitude / deg 45
0
45
90 180 270
Longitude / deg
(a) 300 (b) 90
0 06:30 0 06:30
250
45
200
Latitude / deg
Latitude / deg
30 30
|Bc|/nT
/deg.
06:20 150 06:20 0
100
45
50
06:10 06:10
0 90
150 160 170 180 150 160 170 180
Longitude / deg Longitude / deg
Figure 3: Projections of the orbit of MEX onto the surface of Mars. The two large panels show the MEX
orbit trajectory plotted versus planetographic latitude and longitude (black line), with the intervals
of peak density enhancements highlighted by the white blocks, and the interval for which the epsilon
signature is clearly visible by the black block. The magnitude |B| of the Martian crustal field is shown
underneath the orbit in panel (a), while the inclination angle δ is shown in panel (b). Both parameters
are determined from the Cain et al. [2003] model, evaluated at a fixed 300 km altitude. The upper inset
shows the larger context of the two main panels.
10with near-vertical fields (large positive or negative similar values for fce when instead using a model
δ). The epsilon signature meanwhile is observed of the crustal magnetic field strength at the loca-
on the northern edge of one such longitudinally tion of the spacecraft, as seen by the blue trace in
extended vertical field region, in the transition to Figure 2d.
near-horizontal fields.
No other complete epsilon signatures are ob-
served on this orbit, although several intermit- 3 Summary and Discussion
tent distant reflections are observed following the
Two unusual signatures have been noted in the
other peak density enhancements originally re-
MARSIS data obtained on MEX orbit 2359, dur-
ported by Nielsen et al. [2007]. Additionally, oc-
ing day 318 of 2015. Firstly, significant apparent
casional oblique reflections at frequencies between
enhanced peak plasma densities at ∼130-150 km al-
fpe and fmax are also present, for example the hy-
titudes and associated evidence for large-scale den-
perbolic traces visible in Figure 2b from ∼06:19:00
sity irregularities are noted at three distinct loca-
to ∼06:21:30, and separately from ∼06:21:30 to the
tions on the orbit as the spacecraft passes through
end of the displayed interval. These are commensu-
regions of near vertical crustal magnetic fields (e.g.,
rate with the type of oblique reflections previously
Figure 1b). Nielsen et al. [2007] suggest these are
reported e.g. by Duru et al. [2006], attributed to
due to the localized heating of the ionosphere near
stable ionospheric upwellings. We conclude there-
the peak, possibly as the result of unstable flows
fore that the processes that give rise to both the
resulting ultimately from solar wind driving. Sec-
peak density enhancements and the ducted echoes
ondly, shortly after one such peak density enhance-
are present not just at a single location in Mars’s
ment, ducted propagation of the MARSIS sound-
ionosphere during this period, but at several loca-
ing pulses is observed, indicating the presence of
tions with favourable magnetic field geometry. Fur-
field-aligned density cavities at spacecraft altitudes,
thermore, they occur in addition to other more typ-
in near-horizontal fields (e.g., Figure 1d). Here,
ical and widely reported modifications of the iono-
we have shown that these two phenomena appear
sphere by the presence of intense crustal fields.
linked by continuous signatures in the data, and
Zhang et al. [2016] suggest that the density deple-
take place on the same or similar field lines (noting
tion that gives rise to the ducted propagation and
that the zonal crustal field in this region is rela-
resulting epsilon signature may be artificial in ori-
tively weak, but can still lead to an East-West dis-
gin, with the high power MARSIS sounding pulse
placement in the footprints of these field lines in
rapidly establishing a field-aligned plasma density
the ionosphere).
depletion through the ponderomotive force. This
In Figure 4 we sketch our interpretation of
mechanism has been suggested to be effective in
this event, showing qualitatively the physical iono-
certain situations when sounding the Earth’s iono-
spheric and sounding geometry (a) and the result-
sphere with similar instruments [Benson, 1997], in
ing reflections received by MARSIS (b). Prior to
the case where the local plasma frequency fpe is a
the arrival of the spacecraft an initial density per-
low integer multiple of the cyclotron frequency fce ,
turbation was formed in the deep ionosphere, per-
fpe /fce ≈ n for integer n. Benson [1997] discuss
haps as the result of the ionospheric flows suggested
various examples for which n = 3 and n = 4, but
at low altitudes by Nielsen et al. [2007], indicated
note that a full theoretical description of the cou-
by the dashed portion of the orange horizontal line
pling is lacking. However, as can be seen in Fig-
depicting the altitude of the ionospheric peak.
ure 2e, for this event at least the period in which
As soundings are performed in near-vertical field
the epsilon is most clearly observed has fpe /fce
regions (at the black circle), the nadir reflected
∼100-1000, as first reported by Zhang et al. [2016],
trace is spread over a broad range of delay, as well
somewhat stretching the postulated requirement
as showing the highly enhanced peak densities de-
fpe /fce ≈ n. While the measurement of the local
scribed by Nielsen et al. [2007]. Simultaneously,
cyclotron frequency by MARSIS is increasingly un-
density perturbations are produced along crustal
reliable at these >100 nT values (and clearly aliased
field lines to higher altitudes. Whether these con-
measurements are present, as indicated by the red
sist of depletions or enhancements, the resulting
circles in Figure 2d), we nevertheless obtain very
11Figure 4: a) MEX’s trajectory and MARSIS radar echoes obtained at two locations during a passage
through a crustal field arcade. Thin black lines depict the crustal field, the thick black line MEX’s
trajectory. Black and red circles indicate the location of MEX during two MARSIS soundings, A and
B. Arrowed lines of the same color indicate the path of radar reflections, labeled NA and NB for the
two nadir reflections and T1 and T2 for the two ducted traces seen in sounding B. The horizontal solid
orange line indicates the altitude of the ionospheric peak, drawn dashed in the region where enhanced
peak densities are observed. Meanwhile, the orange shaded region schematically indicates the structure
of the ducting region along the field lines. b) Illustrative features of the two ionograms corresponding
to the locations in a), versus time delay τ and sounding frequency f . The same color code applies for
the two ionograms. The resonance ‘spike’ at the local plasma frequency fpe is present in both cases
(harmonics nfpe omitted for clarity). fOC indicates the low frequency cutoff of the ducted traces.
12structure in the refractive index results in a natural tion of the spacecraft within it.
radio duct indicated by the orange shaded region. If the density irregularities responsible for the
Then, as the spacecraft moves into more horizontal ducted propagation are found at all altitudes down
fields (at the red circle), a relatively ‘clean’ nadir to the ionospheric peak on the same field lines, then
reflection is instead observed from the unperturbed it is reasonable to suggest that the delay-spread
ionosphere below the spacecraft. This is in ad- nadir reflections observed earlier on the orbit could
dition to more complex signatures simultaneously be the result of the same irregularities. We can
detected resulting from ducted propagation of the estimate the perpendicular length scale of the ir-
sounder pulses along the field-aligned irregularities, regularities responsible, as the spacecraft is appar-
including a long-distance duct that leads to reflec- ently contained within the same ducting cavity for
tions from the ionospheric peak, close to the region at least the time taken for a single MARSIS iono-
sampled by the nadir case from the first sounding. gram to be obtained (∼1.5 s of actual sounder oper-
The most obvious signature of these ducted traces ation). Assuming a horizontal field for the duration
is the ‘epsilon’ noted by Zhang et al. [2016]. We of the observed epsilon signature, and perpendicu-
note the apparent lack of the isolated correspond- lar (vertical) velocity of MEX 0.5 km s−1 yields a
ing ‘T1’ trace in Figure 1d, which is expected at minimum perpendicular length scale of ∼750 m.
delays smaller than that of the nadir reflection on Density cavities with smaller length scales would
the basis of the properties of the ‘T2’ and ‘T1+T2’ give rise to intermittent ducted echoes as the space-
traces (from which the expected ‘T1’ trace can be craft enters and exits individual density structures
uniquely determined). This missing ‘T1’ trace may during the course of a single sounding, not consis-
simply be the result of the intense interference at tent with observations.
low delays and low frequencies associated with the Meanwhile, estimates of the spatial extent of the
localised antenna-plasma interaction. We also note cavity along the field line can be obtained by con-
that both the nadir and ducted reflections that sidering the observed time delay of the different
take place in the regions of enhanced peak plasma branches. For the ‘T2’ and ‘T1+T2’ branches de-
density appear to be gradually attenuated toward picted in Figure 1c, these maximum delays are ∼3.1
higher frequencies (Figure 1b), rather than having and ∼3.7 ms, respectively. Utilising the same in-
a sharp maximum frequency above which they are version scheme employed for the nadir reflection
no longer detected. This could indicate that the described by Morgan et al. [2013], the longer (T2)
true density could well be higher than those esti- duct branch can be tentatively estimated to be
mated here. ∼300-400 km in length. While this method may
Zhang et al. [2016] suggest that the low- not be strictly appropriate in this situation, we nev-
frequency cutoff of the epsilon signature, i.e. the ertheless are satisfied that it yields at least a rea-
point at which the three branches merge to a sonable order of magnitude estimate for the extent
single point, is related to the artificial formation of the duct. These are then comparable to the es-
mechanism, although the theory first put forward timated length of the field lines encountered by the
by Benson [1997] requires a highly magnetized spacecraft at this location, again suggesting that
plasma, unlike that present at Mars. We suggest the irregularities responsible for the ducting can be
that it instead is determined purely by the field- present down to the peak of the ionosphere.
perpendicular gradients in the density cavity (or, Recently, MAVEN observations have shown ev-
smaller sub-cavities). The efficiency with which the idence of ionospheric irregularities produced by
duct is able to guide waves will be strongly depen- the Farley-Buneman instability acting in the lower
dent on frequency, and sensitive to the magnitude Martian ionosphere [Fowler et al., 2017], analogous
and gradient in the electron density. Small varia- to the E-region of Earth’s ionosphere. In-situ mea-
tions in these parameters will change the effective surements by instruments on MAVEN show fluctu-
‘opening-angle’ of the duct, and lead to changes in ations in both density and magnetic field in regions
the ducted frequencies [see e.g. Calvert, 1995]. In of near-horizontal fields with similar characteristic
this case, the low frequency cutoff of the duct is scales as those inferred for the ducting field-aligned
then purely a result of the spatial structure of the irregularities studied here. In this instance, there
plasma density within duct, and the varying loca- can be no question that these are a natural fea-
13ture of the Martian ionosphere, as while the Lang- ions and the neutral atmosphere leads to ion heat-
muir Probe instrument on MAVEN does act as a ing, while having little effect on the electron tem-
sounder, transmitting a low power white noise sig- perature. At higher altitudes near the spacecraft,
nal through the booms on which the probes are atomic ions dominate. The ion loss rate is con-
supported, it does so with less than 0.1% of the trolled by the rate of charge exchange with neutral
effective radiated power of MARSIS. Fowler et al. species to form molecular ions, which then quickly
[2017] report density variations of up to ∼200%, dissociatively recombine. Increased ion drift speeds
much greater than the few % typically required in these regions increase the cross-section for charge
to lead to ducted wave propagation in the Earth’s exchange and hence decrease the plasma density,
ionosphere. as observed. Qualitatively similar processes are
For ducted echoes observed by topside sounders widely studied in the Earth’s F-region (upper) iono-
at Earth, it is observed that the process is much sphere [e.g. Killeen et al., 1984].
more efficient for the X-mode rather than the O- The observed ducted propagation of the MAR-
mode, with the ‘opening-angle’ of the duct be- SIS sounding pulses suggests a spatially structured
ing significantly smaller for the O- than the X- plasma in the direction perpendicular to B, and
mode [Muldrew and Hagg, 1969]. This may explain this is expected to reflect the structure of the elec-
in part why ducted echoes are only rarely observed tric field also. The associated enhanced plasma
by MARSIS, as the much weaker typical magnetic drift should therefore be mostly aligned along ap-
field strength encountered at Mars, even in regions proximately constant magnetic flux, i.e. mostly in
of relatively intense crustal fields are so low that O- the East-West direction. Following the discussion
and X-mode effects are rarely, if ever, relevant. We above, we suggest that these drift channels would
also note again that radar spread-F is also routinely have widths of ∼750 m, although the East-West
associated with the presence of ducted echoes [Mul- extent would be much larger and not resolved by
drew and Hagg, 1969]. these (single spacecraft) observations.
Following Nielsen et al. [2007], we suggest that In summary, recently analysed MARSIS data
intense electric fields at lower altitudes near the further suggests that Mars’s highly structured
ionospheric peak are the most likely cause of the crustal fields exert a significant influence on the
observed peak density enhancements, density irreg- structure of the ionosphere on the small scales re-
ularities, and ducted propagation of the sounding quired to affect the propagation of ∼MHz radio
pulses. Such electric fields could conceivably be transmissions. Crustal fields, able to modify iono-
generated due to winds in the neutral atmosphere, spheric plasma flows driven by the solar wind in-
external forcing by the solar wind, gradients in teraction, can lead to large local plasma density
ionospheric conductivity, among other sources. For enhancement, and the generation of field-aligned
sufficiently strong electric fields, the relative drift irregularities at all ionospheric altitudes. Future
between partially demagnetized ions and magne- observations with both MEX and MAVEN are ex-
tized electrons can exceed the local sound velocity, pected to provide much more detailed understand-
causing the onset of plasma turbulence and electron ing of these processes, along with more rigorous
heating. At these altitudes, molecular ions domi- comparison with similar processes in the terrestrial
nate, and are lost by dissociative recombination, ionosphere.
the rate coefficient of which decreases with increas-
ing electron temperature [see e.g. Schunk and Nagy,
2004]. Thus, the net effect is a rapid and localized Acknowledgements
increase in plasma density, as suggested by Nielsen
Work at IRF was supported by grants from the
et al. [2007], with this situation persisting so long
Swedish National Space Agency (DNR 162/14)
as the driving electric field is maintained.
and the Swedish Research Council (DNR 621-
Additionally we suggest that the same intense
2014-5526). Work at Iowa was supported
electric fields should lead instead to a decrease in
by NASA through contract 1224107 from the
plasma density at higher (spacecraft) altitudes, as
Jet Propulsion Laboratory. Work at Boston
seen in the data presented in this paper. Here, fric-
University was supported by NASA award
tion between the (now magnetized) E × B drifting
14NNX15AM59G. All data used in this paper are G. T. Delory, S. W. Bougher, M. H. Acuña,
available in the ESA planetary science archive, and H. Rème (2006), On the origin of aurorae
https://archives.esac.esa.int/psa. on Mars, Geophys. Res. Lett., 330, L01201, doi:
10.1029/2005GL024782.
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