The Impact of Solar Radiation on the Martian Upper Atmosphere S.C. Chakravarty12and Kamsali Nagaraja2 1Indian Centre for Space Physics ICSP Kolkata 700099 Emailsubhascsp.res.in or chakravartysubhasgmail.com Chakravarty

2025-05-06 0 0 1.03MB 16 页 10玖币

The Impact of Solar Radiation on the Martian Upper Atmosphere

S. C. Chakravarty1,2and Kamsali Nagaraja2

1Indian Centre for Space Physics (ICSP), Kolkata 700099, Email:subhas@csp.res.in or chakravartysubhas@gmail.com (Chakravarty)

2Department of Physics, Bangalore University, Bengaluru 560056, Email:kamsalinagaraj@bub.ernet.in (Nagaraja)

Abstract

The ﬁrst in-situ measurements of the altitude proﬁle of Martian upper atmospheric density and com-

position were carried out by the Viking lander missions in 1976. The MAVEN and MOM spacecraft

launched in September 2014 with mass spectrometers and solar radiation measuring payloads have vastly

expanded this initial data base. Using a rare set of near-simultaneous data from these two orbiters, we ﬁnd

that there is either an increasing (e.g., for 𝐶𝑂2and 𝐴𝑟) or a decreasing (e.g., for 𝑂) trend of the density

proﬁles by a factor of 2 between June 1 to June 15, 2018 in the height region of ∼150-300 km. A time

series analysis of the concurrent in-situ solar EUV spectral ﬂux and the 𝐻+ion velocities of the incident

solar wind measured near MAVEN periapsis showed the former going through a decrease of only ∼10%

compared to the latter’s decrease by a factor of 4 within the same non-solar-ﬂare period of observation.

The estimates of standard errors and the use of the linear regression analysis for the correlation coeﬃcients

between densities and solar radiation components have been carried out. Invoking simple photochemical

equilibrium conditions with the dissociation of 𝐶𝑂2(producing 𝐶𝑂 and 𝑂) through solar EUV radiation

and the solar wind 𝐻+ion impact process, the day-to-day variations of these constituents are estimated. The

high and signiﬁcant anti-correlation between the density variations of 𝐶𝑂2and 𝑂due to the dissociation

of 𝐶𝑂2by the solar wind particle radiation is clearly demonstrated. The cause for the increasing densities

of 𝐴𝑟 like that of 𝐶𝑂2during this period is more complex and would likely be governed by the temperature

variations due to absorption of solar EUV/charged particle radiation and other interacting dynamical eﬀects.

Keywords: Planetary atmosphere, Martian thermosphere/exosphere, solar EUV, solar wind plasma,

MOM/MENCA, MAVEN/NGIMS

1 Introduction

Considerable progress has been made to conduct in-situ observations of Martian surface and atmospheric

parameters using orbiters, landers and rovers. Near-surface meteorological data has been analysed and

consolidated in diurnal, seasonal and inter-annual variations [1]. However, till recently, the measurements

of upper atmospheric composition and density of Mars have been limited to the two sets of observations

taken by the Viking landers while traversing down through its thin atmosphere [2-4]. In September 2014,

the Mars Atmosphere and Volatile Evolution (MAVEN) and the Mars Orbiter Mission (MOM) spacecraft

entered Martian orbit and were placed in elliptical orbits around Mars with one of the main objectives to

gather substantial data on spatial and temporal proﬁles of various upper atmospheric neutral/ion density

and composition parameters [5,6].

Mars has a well-mixed region of the homosphere with the homopause at ∼120 km altitude. The

thermosphere extends above 120 km ﬁnally merging into the exosphere. From the exosphere the lighter

gases may escape due to negligible neutral gaseous collisions and through interaction with solar EUV

and energetic charged particle radiations. This loss process usually starts from around 220 km, called the

exobase [7,8]. With the availability of continuous thermosphere/exosphere density data (due to its lower

perigee) for more than one Martian year from MAVEN, it is possible to study the eﬀect of solar forcing on

neutral densities [9].

The results obtained from both MAVEN using NGIMS (Neutral Gas and Ion Mass Spectrometer)

payload and MOM using MENCA (Mars Exospheric Neutral Composition Analyser) payload have so far

arXiv:2210.01417v3 [physics.space-ph] 13 Sep 2024

provided basic information about the spatial variation of the upper atmospheric gas constituents and ion

species delineating their vertical and horizontal distributions [10-12]. Recently Sarris [13] has suggested

that short term neutral density changes in Earth’s thermosphere could be attributed to the variations of

solar EUV as well as to the solar particle radiation. In the absence of an earth-like magnetosphere, the

charge particle interaction with Martian atmospheric constituents would need further studies.

MAVEN has many instruments which measure solar wind parameters along its track covering the

lower altitude range around the periapsis (∼150-300 km) of MAVEN, which is the main height region

of interest in this paper. In one day, this region near perigee is covered ∼5 times and the average ﬂux

received each day depends on the daily variation of solar activity. The primary purpose of this paper is to

explore the non solar-ﬂare variation of diﬀerent thermosphere-exosphere gas constituents of Mars by using

NGIMS/MAVEN and MENCA/MOM data and to assess the role of the variable solar energetic radiation

as a possible cause.

2 Dataset and Method of Analysis

In this study, we mainly consider the region of the Martian atmosphere between the upper thermosphere and

lower exosphere (∼150-300 km) with the exobase level at ∼220 km. This region is identiﬁed as the space

where the interaction of solar EUV and charged particle radiation with various atmospheric constituents

takes place. Enhanced solar radiation levels may lead to the escape of 𝐻and 𝑂, due mainly to the relatively

weak surface gravity of Mars as compared to that of the Earth [14].

While both spacecraft measured the upper atmospheric composition and densities, their respective

spatial and temporal coverage are quite diﬀerent with reference to the altitudes of interest. We could only

select the period of June 2018 with near simultaneous observation in the height range of interest. Such a

coincidence of getting near-simultaneous observations is very rare and has happened for the ﬁrst time.

Further, the period of only June 1-15, 2018 has been selected and the second half of June 2018 avoided,

which was aﬀected by the Planet Encircling Dust Event (PEDE). The eﬀect of the global dust storm

on thermospheric densities has been studied using the available MENCA and NGIMS data for the event

during the second half of June 2018 which demonstrated the asymmetry between the daytime and nighttime

thermospheric density observations of both the spacecraft [15]. In an earlier study we have already shown

the highly sensitive response of the neutral atmospheric composition and density to an eruptive event

of coronal mass ejection (CME) using MENCA data for December 2015, when MAVEN data was not

available [12].

The results in this paper are based on the solar quiet time vertical atmospheric density proﬁles of

constituents, 𝐶𝑂2,𝑂,𝐴𝑟,𝑁2etc., derived from similar mass spectrometric instruments carried by both

the spacecraft. As the time interval between two successive observations is a few days for MOM and only

a few hours for MAVEN, we have mainly used MAVEN data for better statistics.

2.1 MENCA Instrument

MOM arrived at Mars on 24 September 2014 in an eccentric orbit of ∼422 km×76,993 km with an orbital

period of about 72 h. During December 2014, orbital manoeuvres brought down the periapsis altitude to

around 263 km. The MENCA observations measure total atmospheric pressure and partial pressures of

various atmospheric constituents covering 1-100 amu. More details about the instrumentation, limitations,

observation errors and sources of contamination can be found in Bhardwaj et al. [16-18].

The data from this experiment has been made available for the project from time to time through the

ISRO Space Science Data Centre (ISSDC). It consists of total pressure and partial pressure values as

counts in ampere units with a variable time resolution of 12-48 s near periapsis. The inbound and outbound

trajectories cover the lowest altitudes. Before this base-level data can be used for scientiﬁc studies, further

processing has been done by us, which include instrument calibration to convert the raw current counts to

pressure unit Torr, background correction with respect to higher altitude measurements, temporal/spatial

smoothing and tagging each observation point with ancillary data such as latitude, longitude, altitude and

solar zenith angle.

The data processing for each orbit involves handling of a number of ﬁle pairs, each containing the total

atmospheric pressure and partial pressures of constituents for diﬀerent time blocks. The ﬁles for partial

pressures contain the data in a continuous time-sequenced array form, which are converted into a tabulated

columnar format with a time of observation in UTC, corresponding to the nine values of partial pressures

for each constituent from 1 to 100 amu and total pressures synchronised in time. ISSDC also provides

the speciﬁc SPICE (Spacecraft Planet Instrument C-matrix Events) kernel ﬁles to extract the local solar

time (LST), altitude, latitude, longitude and solar zenith angle values linked to each record of observation.

The calibrated partial pressure values in Torr along with the associated ephemeral and spatial information

for each orbit have been used in analysing the results. After the above treatment of basic data, the MOM

orbit-wise analysis is carried out to select the useful partial pressure values from the periapsis altitude to

about 500 km.

2.2 NGIMS and other Instruments onboard MAVEN

The NGIMS instrument of the MAVEN spacecraft has been utilised to determine the density and com-

position of the upper atmosphere’s neutral and ionic species in a range of 2 to 150 amu [19]. NGIMS

science-mode observation is conducted between 500 km to the periapsis altitude of ∼150 km during each

orbit lasting 1200 s with a vertical resolution of ≈2 km [20]. The level-2 (version-8 and revision-1) datasets

of NGIMS, retrieved from MAVEN Science Data Center are used for this study. The Extreme Ultra Violet

Monitor (EUVM) instrument on MAVEN measures the solar irradiance at Mars using three photometers

sensitive to the wavelengths 0.1-7 nm, 17-22 nm and 121.6 nm [21].

The Solar Energetic Particle (SEP) instrument on MAVEN consists of two dual, double-ended solid-

state telescopes with four look directions per species, optimised for parallel and perpendicular Parker Spiral

viewing of energetic ions (25 keV to 12 MeV) and electrons (25 keV to 1 MeV) with 1 s time resolution.

SEP can measure energy ﬂuxes that range from 10 to 106eV/(cm2s sr eV) [22].

The Solar Wind Ion Analyzer (SWIA) measures the energetic ions from the upstream solar wind and

magnetosheath around Mars, within the energy range of 5-25000 eV q−1and an angular range of 360×90

deg [23]. During its nominal operation, pointing to the Sun from the top corner deck of the MAVEN

spacecraft, the SWIA instrument measures the nominal solar wind ﬂows. This instrument provides high-

resolution ion velocity measurements with a broad energy spectrum. We utilize the hourly mean velocity

data from the SWIA instrument for the period June 3-15, 2018 representing the average condition of these

velocity spectra through the altitudes of interest.

3 Results and Discussion

The daytime mean density proﬁles of 𝐴𝑟 measured by both the spacecraft on a few selected days during June

2018 are shown in ﬁgure-1. The proﬁle shapes and a broadly increasing trend of density values between

June 1-15, 2018 measured by both NGIMS and MENCA compare quite well as can be seen between the

160-220 km height range within the limitation of diﬀerent sampling rates and height resolutions. In the

absence of any solar energetic event or any eﬀect of dust storm sweeping the lower atmosphere during

this time, such a systematic and large variation within a short period of 2 weeks is diﬃcult to explain.

For both sets of data the changes in Solar Zenith Angles (SZA) are only of the order of a few degrees

during the observation period covering a small arc of the orbital path near the periapses. This is illustrated

through ﬁgure-2 and ﬁgure-3 for MOM and MAVEN trajectories respectively. Hence an average density

variation up to a maximum of 10% can only be explained due to the SZA eﬀect. The observed increase

of proﬁle densities by a factor of 2-3 (derived from ﬁgure-1b) would mean a thermospheric temperature

Figure 1: Mean daytime density proﬁles of 𝐴𝑟 in the 160-220 km of the upper atmosphere of Mars as measured by a) MENCA

and b) NGIMS for a few selected days between June 1-15, 2018. The MENCA data consists of processed mean values of partial

densities of gas constituents with a height resolution of 5 km and the NGIMS data is obtained from the MAVEN Science Data

Centre. Here the daily mean 𝐴𝑟 concentrations are computed from 5 daytime passes with a height resolution of 2 km.

change from 250 K to 500 K (estimated using the relation H = kT/mg, where H is the scale height, k

the Boltzmann constant, g the gravitational acceleration of Mars and m the molar mass of 𝐴𝑟, and the

hydrostatic equilibrium relation 𝑁=𝑁𝑜exp −(𝑧−𝑧𝑜)

𝐻; where N𝑜and N are number densities of 𝐴𝑟 at

heights z𝑜and z respectively). Many authors have computed the mean values of temperatures from scale

heights in order to explain the density variations [18, 20, 24]. Bharadwaj et al. [18] measured 𝐴𝑟 density

using MOM data and attributed sharp bends in scale height proﬁles due to thermospheric temperature

changes which can explain the variations in 𝐴𝑟 density. In our results, the absence of any sharp change

in the slope of the 𝐴𝑟 density proﬁles (ﬁgure-1) does not indicate a major temperature variation of the

required magnitude. The small ﬂuctuations within the individual vertical proﬁles point to other possible

dynamical eﬀects, such as the tidal oscillations, planetary and gravity waves propagation [25].

Along with the SZA, LST and latitudinal variation, ﬁgure-3 also shows the typical proﬁles of some

of the major atmospheric constituents from one of the daytime passes of MAVEN on 3 June 2018. The

plots show the measured densities along a small trace of the orbit near its periapsis (∼160-300 km). The

measured values are shown for both the inbound and outbound parts of the orbital trace. For comparison

with diﬀerent days, we have used the densities obtained from the inbound part for further analysis. During

this coverage, the LST changes only by about 40 min and SZA by a few degrees. Plots of densities of 𝐶𝑂2,

𝑂,𝐶𝑂 and 𝑁2are shown reaching peak values near 160 km, i.e., the periapsis altitude of MAVEN. In view

of the interesting result on the day-to-day variation of thermosphere/exosphere of 𝐴𝑟 densities as shown

in ﬁgure-3, a similar analysis is carried out for other constituents by using only the MAVEN data having

more observation points with 4-5 daytime passes per day during June 1-15, 2018. These density proﬁles of

the major gas constituents on diﬀerent days of June 2018 are shown in the ﬁgure-4. It can be seen that all

the gas concentrations show day-to-day variations which is very striking for 𝐶𝑂2and 𝑂. A gradual rise of

the 𝐶𝑂2-𝑂cross-over altitude from ∼200-230 km between June 3-13, 2018 is quite clear from the ﬁgure.

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TheImpactofSolarRadiationontheMartianUpperAtmosphereS.C.Chakravarty1,2andKamsaliNagaraja21IndianCentreforSpacePhysics(ICSP),Kolkata700099,Email:subhas@csp.res.inorchakravartysubhas@gmail.com(Chakravarty)2DepartmentofPhysics,BangaloreUniversity,Bengaluru560056,Email:kamsalinagaraj@bub.ernet.in(Nagara...

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The Impact of Solar Radiation on the Martian Upper Atmosphere S.C. Chakravarty12and Kamsali Nagaraja2 1Indian Centre for Space Physics ICSP Kolkata 700099 Emailsubhascsp.res.in or chakravartysubhasgmail.com Chakravarty

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