Submitted for publication in JGR -Planets 1 Effects of Solar Activity Solar Insolation and the Lower

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Submitted for publication in JGR-Planets

Effects of Solar Activity, Solar Insolation and the Lower

Atmospheric Dust on the Martian Thermosphere

N. V. Rao1*, V. Leelavathi2, Ch. Yaswanth1, Anil Bhardwaj3, S. V. B. Rao2

1National Atmospheric Research Laboratory, Gadanki, India

2Department of Physics, S. V. University, Tirupati, India

3Physical Research Laboratory, Ahmedabad, India

* Corresponding author: nvrao@narl.gov.in

Key points

• A diagnosis of MAVEN observations revealed the signatures of solar activity, solar

insolation and dust effects on the Martian thermosphere

• A regression analysis is used to quantify the dominant variabilities in the Martian

thermospheric temperatures and densities

• Global dust storms rise the thermospheric temperatures by ~22-38 K and enhance the

hydrogen escape fluxes by 1.67-2.14 times

Abstract

A diagnosis of the Ar densities measured by the Neutral Gas and Ion Mass Spectrometer

aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) and the temperatures derived

from these densities shows that solar activity, solar insolation, and the lower atmospheric dust

are the dominant forcings of the Martian thermosphere. A methodology, based on multiple

linear regression analysis, is developed to quantify the contributions of the dominant forcings

to the densities and temperatures. The results of the present study show that a 100 sfu (solar

flux units) change in the solar activity results in ~136 K corresponding change in the

thermospheric temperatures. The solar insolation constrains the seasonal, latitudinal, and

diurnal variations to be interdependent. Diurnal variation dominates the solar insolation

variability, followed by the latitudinal and seasonal variations. Both the global and regional

dust storms lead to considerable enhancements in the densities and temperatures of the Martian

thermosphere. Using past data of the solar fluxes and the dust optical depths, the state of the

Martian thermosphere is extrapolated back to Martian year (MY) 24. While the global dust

storms of MY 25, MY 28 and MY 34 raise the thermospheric temperatures by ~22 – 38 K, the

regional dust storm of MY 34 leads to ~15 K warming. Dust driven thermospheric temperatures

alone can enhance the hydrogen escape fluxes by 1.67-2.14 times compared to those without

the dust. Dusts effects are relatively significant for global dust storms that occur in solar

minimum compared to those that occur in solar maximum.

Submitted for publication in JGR-Planets

Plain Language Summary

Understanding the Mars thermospheric (altitude, ~100 km – 220 km) variability is

constrained by unambiguously distinguishing the effects of solar activity (variations in the solar

irradiance at the Sun) and dust forcings from those of solar insolation (the amount of solar

irradiance received at the planet). In the present study, we used the Mars upper thermospheric

densities and temperatures measured by the Mars Atmosphere and Volatile Evolution

(MAVEN) Mission. Supported by the MAVEN observations, we developed a methodology

that successfully isolates the contributions of the solar activity, solar insolation and the lower

atmospheric to the thermospheric temperatures and densities. An increase in the solar activity

from solar minimum to solar maximum increases the thermospheric temperatures. The solar

insolation drives the seasonal, diurnal and latitudinal variations in the Martian thermosphere.

Diurnal and latitudinal variations dominate the seasonal variations. While global dust storms

raise the thermospheric temperatures by 22-38 K, regional dust storms lead to ~15 K warming.

Heating of the thermosphere by the global dust storms enhances the thermal escape of hydrogen

at the exobase by 1.67-2.14 times. The relative importance of the global dust storms to

hydrogen escape flux increases with decrease in solar flux.

1. Introduction

The thermosphere of Mars (altitude, ~100 – 200 km) is a reservoir of volatile species

and hence the thermal and dynamical state of this region significantly affects gaseous escape

from the planet. Therefore, understanding the processes that contribute to the energetics and

dynamics of the Mars thermosphere is extremely important (e.g., Bougher, Cravens, and

Grebowsky et al., 2015). Since the thermosphere of Mars lies at intermediate altitudes, it

responds promptly to the energetic inputs from the Sun and is intimately coupled to the lower

atmospheric processes. Heating by the solar extreme ultraviolet (EUV) and X-ray radiation is

the main energetic input to the thermosphere from above whereas seasonally varying dust and

waves are the primary contributors from below. However, rotation of the planet, its obliquity

and eccentricity of the ecliptic result in spatiotemporal variations in the solar insolation that the

planet receives. These in turn are expected to cause the diurnal, latitudinal, and seasonal

variations in the state of the thermosphere. In addition, meridional circulation also changes in

the thermospheric temperatures through adiabatic heating and cooling in regions of

convergence and divergence of winds, respectively (Bougher, Pawlowski, and Bell et al., 2015;

Elrod et al., 2017). The meridional circulation is characterized by a two-cell pattern in

equinoxes and a single-cell pattern in solstices. Thus, the state of the thermosphere is a result

of balance between several processes that heat, cool, and redistribute energy (Bougher et al.,

1999; Bougher, Pawlowski, and Bell et al., 2015; Medvedev and Yiğit, 2012). All these

forcings are expected to drive the thermospheric temperatures and densities.

Spacecraft measurements reported in previous studies have constrained the

thermospheric temperatures in the range of 150 K – 300 K (e.g., Bougher et al., 2017; Stone et

al., 2018). Spacecraft measurements are generally made at varying altitudes, latitudes and local

times and contain contributions from several time-varying external forcings (that are described

in the previous paragraph) that affect the thermosphere. As a result, spacecraft measurements

Submitted for publication in JGR-Planets

display a complex variability pattern (Bougher et al., 2017; Forbes et al., 2008; Jain et al., 2021;

Rao et al., 2021; Stone et al., 2018). Quantifying the response of the Martian thermosphere to

these time varying external forcings has been a difficult task due to the inherent complexity of

the spacecraft measurements. The coarser spatial and temporal resolutions of the spacecraft

measurements often demand several months to years of data to properly understand the

dominant variabilities. Inter-annual and other long-term variabilities further complicate the

problem.

Using temperatures derived from precise orbit determination of the Mars Global

Surveyor (MGS), Forbes et al. (2008) quantified the solar flux and seasonal contributions to

the Mars exospheric temperatures. Their observations were, however, confined to a constant

local solar time (LST) and a narrow latitude range (40°S to 60°S), which made the estimation

of the solar flux contribution easier. The contribution of the lower atmospheric dust, however,

could not be quantified in their study. This is because the growth phase of the 2001 global dust

storm (GDS) occurred contemporaneously with the rising phase of the solar maximum and

hence the effects of the two forcings could not be distinguished (Forbes et al., 2008). In recent

years, measurements by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft

provided the longest record of thermospheric densities and temperatures on Mars. Due to the

slow precession of MAVEN’s periapsis, these measurements span a wide range of latitudes

and LST and were obtained under varying solar flux and lower atmospheric dust conditions.

As a result, the temperatures obtained from MAVEN measurements display a complex

variability pattern (Bougher et al., 2017; Jain et al., 2021; Rao et al., 2021; Stone et al., 2018).

Contrary to the MGS measurements (Forbes et al., 2008), the MAVEN measurements (that are

used in the present study) were made in the medium to low solar activity period. This period

also witnessed a GDS starting from June 2018 (solar longitude, Ls=185° in MY 34). The

occurrence of a GDS during the declining phase of solar activity and the slow precession of

MAVEN’s periapsis in latitude and LST provide an unprecedented opportunity to isolate the

contributions of various forcings that affect the Martian thermosphere. This particular aspect

is addressed in this study and a regression analysis is used to quantify the contributions of the

dominant forcings. The dust driven thermospheric temperatures are further used to assess their

relative importance in the hydrogen escape.

The organization of the paper is as follows. Section 2 describes the NGIMS instrument

and the method of estimation of the thermospheric temperatures from NGIMS density

measurements. In section 3, we first diagnose the NGIMS observations for signatures of

possible drivers of the Mars thermospheric variability and develop a methodology to quantify

the contributions of these drivers. Using this method, we estimate the contributions of the

dominant forcings to the thermospheric temperatures and densities and quantify the diurnal,

seasonal, and latitudinal variations. In addition, we use the past data of the dust optical depths

and F10.7 cm flux to predict the state of the Martian thermosphere from MY 24 to MY 35.

Finally, we assess the role of the dust driven temperatures in the relative escape of hydrogen at

the exobase. Sections 4 and 5 present the discussion and conclusions, respectively.

Submitted for publication in JGR-Planets

2. MAVEN, NGIMS, and Data

MAVEN was placed in the Martian orbit with (nominal) periapsis and apoapsis

altitudes of ~150 km and ~6200 km, respectively and an inclination angle of ~ 75o, resulting in

an orbital period of ~ 4.5 h. During the so-called “deep dip” campaigns, however, MAVEN’s

periapsis was brought down to altitudes well below (to as low as ~120 km) its nominal periapsis

(Bougher, Jakosky, and Halekas et al., 2015; Jakosky et al., 2015). There were nine such

campaigns during the period of observation considered in the present study and each of these

campaigns lasted for approximately one week. Data from these campaigns are included in the

present study. The main objectives of the MAVEN mission, among others, are to measure the

composition and structure of the Mars upper atmosphere, determine the processes responsible

for controlling them and to estimate the rate of gaseous escape from the top of the atmosphere

to outer space (Jakosky et al., 2015). To achieve these goals, MAVEN is equipped with a suite

of nine instruments that measure several parameters of the Mars upper atmosphere and its near

space environment. One such instrument on MAVEN is the Neutral Gas and Ion Mass

Spectrometer (NGIMS). NGIMS is a dual-source quadrupole mass spectrometer designed to

measure both the neutral and ion densities in the m/z range of 2–150 amu, with unit mass

resolution (Mahaffy et al., 2014). NGIMS is typically operated when the spacecraft’s altitude

is below 500 km (Mahaffy et al., 2014; 2015). In the present study, we use the argon (Ar)

densities measured in inbound segments of the MAVEN’s periapsis passes.

In the present study, temperatures of the Martian thermosphere are computed from the

Ar density profiles. The method of computation similar to that used by Snowden et al. (2013)

for deriving the temperatures of the Titan’s upper atmosphere and subsequently adopted for

Martian studies by Stone et al. (2018) and Leelavathi et al. (2020). For each MAVEN’s inbound

segment, the temperature at the upper boundary is computed by fitting to the top of the

atmosphere (densities between 1×104 and 4×105 cm-3) an equation of the form

N = N0 . exp[

 



] ------- (1)

where N0 is the density at the lower boundary of the fitted region and ro is the distance from the

centre of the planet to the lower boundary of the fitted region, r is the distance from the centre

of the planet, N is the number density, m is the mass of Ar, G is the gravitational constant, K is

Boltzmann constant and M is the mass of Mars. The partial pressure at the upper boundary ()

is then computed using the ideal gas law, =NKT. From the hydrostatic equation, the pressure

at a given altitude is given by

=+





 ------ (2)

Considering this pressure profile, the temperature at each altitude is then obtained by using an

ideal gas law. The upper thermospheric temperatures used in the present study correspond to

the average of the top 10 km of each temperature profile and have an uncertainty of ~5%. In

general, the upper portion of each temperature profile is close to the isothermal nature. Though

the main focus of the study is on thermospheric temperatures, neutral densities are also

presented for completeness.

Submitted for publication in JGR-Planets

In the present study, we use the NGIMS data obtained between February 2015

(MAVEN orbit # 767; Ls=295.4o of MY 32 (MY: Martian Year)) and August 2020 (orbit #

11908, Ls=236o of MY 35). We use Level 2, version 08, revision 01 data of the NGIMS

database (Benna & Lyness, 2014). In addition, we use F10.7 cm solar flux measured at Earth

as a proxy for solar activity and the regularly kriged data of 9.3 µm infrared column dust optical

depth at 610 Pa (denoted as τ) as a proxy for the Mars lower atmospheric dust activity

(Montebone et al., 2015; Montebone et al., 2020).

3. Results

3.1. Diagnosis of the MAVEN observations for dominant variabilities

Figure 1. Ls variation of (a) F10.7 cm solar fluxes (sfu: solar flux units) corrected for the Martian orbit

(orange line) and corrected for a constant heliocentric distance of 1.66 au (black line), (b) τ, (c) Ar

densities at 200 km, and (d) thermospheric temperatures, along with the (e) latitudes of MAVEN

periapses. The MAVEN trajectory is colour coded with LST. Dashed blue vertical lines in all panels

enclose different Martian years as indicated in the bottom panel. Furthermore, the τ values shown in

Figure 1b are obtained by averaging the τ values over all longitudes and within ±10° centered on the

MAVEN’s periapsis latitude for each orbit. To remove the short-term fluctuations, the parameters

shown in (a)-(d) are smoothed over 1° Ls. Note that although the Ar densities and temperatures are

shown as a function of Ls, part of their variability is caused by variation in latitude and LST. Ls= solar

longitude, τ= column dust optical depth; LST=local solar time; MAVEN=Mars Atmospheric Volatile

EvolutioN.

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SubmittedforpublicationinJGR-Planets1EffectsofSolarActivity,SolarInsolationandtheLowerAtmosphericDustontheMartianThermosphereN.V.Rao1*,V.Leelavathi2,Ch.Yaswanth1,AnilBhardwaj3,S.V.B.Rao21NationalAtmosphericResearchLaboratory,Gadanki,India2DepartmentofPhysics,S.V.University,Tirupati,India3PhysicalRes...

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