TP5
Session assets
INTRODUCTION
Venus lacks an intrinsic magnetic field, exposing its upper atmosphere directly to the solar wind. The solar Extreme UltraViolet (EUV) radiation ionizes the upper layer of the atmosphere, known as the ionosphere, which in turn deflects the solar wind around the planet, resulting in the formation of an induced magnetosphere (Futaana et al., 2017). The induced magnetosphere contains different plasma boundaries, for example, the bow shock (BS), where the solar wind is slowed, heated and deflected as it transitions from the supersonic and super-Alfvénic regime to the subsonic and sub-Alfvénic regime. Another boundary, the ion composition boundary (ICB) marks the separation between solar wind and planetary ions (e.g., Martinecz et al., 2008)
The Pioneer Venus Orbiter (PVO) and Venus Express (VEX) missions have provided insights into Venus' plasma environment (e.g., Phillips et al., 1991; Svedhem et al., 2007). Data from these missions reveal that the boundaries of Venus' plasma environment vary in response to various upstream conditions. The BS and the ICB are influenced by the solar cycle phase, the magnetosonic Mach number, the solar wind dynamic pressure, and the Interplanetary Magnetic Field (IMF) direction and magnitude (e.g., Alexander et al., 1985; Russell et al., 1988; Zhang et al., 1991; Russell et al., 2006). In this study, we seek to develop refined boundary models which take into account upstream conditions.
DATASET AND METHOD
To characterize the Venus-solar wind interaction, the key information is how the boundaries' positions depend on various solar wind parameters. Formulating these variations in analytical form allows us to predict their positions. We used boundary crossings and upstream conditions recorded by the ion mass analyzer (IMA), the electron spectrometer (ELS), and the magnetometer (MAG) onboard VEX (Signoles et al., 2023) (Figure 1). We modelled the BS with a conic section curve, symmetrical around the longitudinal axis, expressed by
d = L / (1 + εcosα),
where d and α are the radial and angular coordinates, respectively. The semi-latus rectum L and the eccentricity ε are the free parameters, and we investigated their dependencies on upstream conditions. We also characterized the dayside ICB with a half sphere with radius ρ as free parameter. The nightside ICB is instead best modelled with a linear relation (eg., Martinecz et al., 2008). However, the higher number of crossing points on the dayside (Figure 1) restricted which characteristics of the boundaries we could study. The eccentricity ε in the BS model mainly affects the nightside flaring angle, so we couldn't reasonably investigate its dependence on upstream conditions. Therefore, our study focused solely on variations in L. Additionally, we didn't have enough crossings to model the nightside ICB, so we only considered the dayside portion.
Figure 1: The BS and ICB positions and shapes. The X-axis points in the Sun direction, while R=(Y2 + Z2)1/2. The outer and inner curves represent the BS and ICB derived in this study, respectively. The boundaries' free parameters for the general models are L= 1.525 RV (Venus radius), ε = 1.036 and ρ = 1.122 RV. The BS focus is at x0 = 0.688 RV.
RESULTS AND DISCUSSION
Figure 1 shows the crossing points used in this study and the obtained BS and ICB models using the whole dataset. We further studied the free parameters' variation with different upstream conditions. Figure 2 shows an example of the L variation with IMF magnitude B and mass flux p.
Figure 2: The BS semi-latus rectum L as a function of IMF magnitude and mass flux. Each dot represents a data bin, containing several crossings. The horizontal bars represent the variability of the upstream conditions in each bin and the vertical bars represent the standard errors on L. The quantities are normalized by their average values.
We developed our final BS and ICB model by combining upstream conditions that minimized errors in predicting the boundaries' positions and shapes. In the BS model, the parameter L exhibits a linear dependence on the IMF magnitude and the shock normal angle θbn, while it inversely correlates with the mass flux. The shock normal angle is the angle between the IMF direction and the local shock normal at the crossing point. The ICB model depends linearly on the EUV flux FEUV and is inversely proportional to the energy flux FE. The models improve the accuracy of predicting boundary positions by approximately 22% for the BS and 8% for the ICB, compared to statistical models which disregard upstream conditions. Our findings indicate that the BS is more variable than the ICB as well as more sensitive to upstream conditions. Our conclusions agree with previous studies which suggest that the boundaries are influenced by the IMF, the solar wind dynamic pressure and the solar flux (e.g., Alexander et al., 1985; Russell et al., 1988; Zhang et al., 1991; Russell et al., 2006). Moreover, the shock normal angle dependence creates a quasi-perpendicular/quasi-parallel BS asymmetry. The quasi-parallel side of the BS (θbn< 45°) tends to be closer to the planet compared to the quasi-perpendicular side (θbn > 45°). This behaviour was confirmed in other studies as well (Zhang et al., 1991; Chai et al., 2014; Signoles et al., 2023).
REFERENCES
Alexander, C. J. et al. 1985, Geophys. Res. Lett., 12, 369
Chai, L. et al. 2014, JGRA, 119, 9464
Futaana, Y. et al. G. J. 2017, SSRv, 212, 1453
Martinecz, C. et al. 2008, PSS, 56, 780
Phillips, J. L. et al. 1991, SSRv, 55, 1
Russell, C. T. et al. 1988, JGRA, 93, 5461
Russell, C. T. et al. 2006, PSS, 54, 1482
Signoles, C. et al. 2023, ApJ, 954, 95
Svedhem, H. et al. 2007, PSS, 55, 1636
Zhang, T.-L. et al. 1991, Geophys. Res. Lett., 18, 127
How to cite: Rollero, U., Futaana, Y., and Rojas Mata, S.: Comprehensive Venus boundaries model, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-80, https://doi.org/10.5194/epsc2024-80, 2024.
Introduction
Auroras are hallmarks of the interaction between solar particles and the atmosphere of planets. Martian aurora was first discovered in 2005, since then, four different types have been identified: localized discreet aurora (Bertaux et al., 2005), global diffuse aurora (Schneider et al., 2015), dayside proton aurora (Deighan et al., 2018), and large-scale sinuous aurora (Lillis et al., 2022). All previous detections have been made in the UV from orbit. Here we present, from observations with the SuperCam and MastCam-Z instruments on the Mars 2020 Perseverance rover, the first detection of aurora from the Martian surface and the first detection of the green 557.7 nm atomic oxygen auroral emission on Mars. This is the same emission line that is familiar from terrestrial aurora.
Charged particles accelerated by interplanetary coronal mass ejections (ICMEs) or solar flares are referred to as solar energetic particles (SEPs) (Reames, 1999). Diffuse aurora is strongly correlated with SEP events. ICME-accelerated SEPs travel nearly radially, as opposed to flare-accelerated SEPs which follow the Parker spiral. If the solar source region is identified, ICME-accelerated SEP events at Mars, and thus diffuse aurora, can be forecasted.
The dynamic nature of rover planning and operations allows for a reactive observation strategy that takes advantage of such forecasts. We made several attempts, starting in May 2023, to react to SEP events and observe with the M2020 rover (Farley et al., 2020) instruments at times when we believed the likelihood of emission to be highest. Our fourth attempt, in March 2024, yielded the positive detection reported here.
Instruments
On the M2020, SuperCam and MastCam-Z have the best combination of sensitivity and operational flexibility for detecting diffuse aurora.
SuperCam is a multi-technique spectrometer. Its channel covering the 535-853 nm range includes an optical intensifier which can amplify weak optical signals (Wiens et al., 2021) and therefore make auroral emissions, especially the atomic oxygen forbidden transitions at 557.7 and 630 nm, detectable. Other channels lack an intensifier and therefore the UV emissions commonly seen in Mars aurora (e. g. (Schneider et al., 2015)) are not expected to be detectable.
MastCam-Z is a pair of multispectral, stereoscopic imaging cameras capable of providing broadband and narrowband color images, and direct solar images using neutral density filters. Each camera has a CCD detector with 1648×1214 pixels with a selectable field of view ranging from 6.2°×4.6° to 25.6 ×19.2° (Bell et al., 2021).
Results
On March 15th 2024, a long-lasting C4.9 flare with an accompanying ICME erupted from Active Region 13599. Mars was expected to be centered in the projected path, with ion density peaking around 04:00 UTC, March 18th.
Error Detection And Correction (EDAC) counters are cumulative housekeeping logs, registering bit-flips in spacecraft memories. EDACs are sensitive to SEP events, which manifests as jumps in the otherwise steadily increasing counter. Mars Express logged 4 times as many errors when the ICME was expected to reach Mars compared to before and after the solar storm, as indicated in Figure 1.
Figure 1: Housekeeping error log from Mars Express during the six week interval surrounding the March 18th event. The blue line shows the cumulative errors, the orange line shows the number of errors per day, and the green line indicates the time of the rover observations.
SuperCam and MastCam-Z observations were scheduled shortly after the predicted ion density peak. The SuperCam observation consisted of 2x75 spectra and began at 00:33 local mean solar time (LMST) on Sol 1094, or 06:47 UTC on March 18. Figure 2 shows an average of all spectra, after detector background subtraction, and further processing with a 3-pixel-wide median filter. Fitting a gaussian line-shape (0.452 nm FWHM) and 3rd-order polynomial continuum, we obtain 93 R for the 557.7 nm auroral emission with a 1-σ uncertainty of 13 R (Rayleigh is a photon flux, where 1 R corresponds to a column emission rate of 1010 photons per square metre per column per second). No other emission lines were observed. From preliminary radiative-transfer modeling, using an adapted version of the multiple-scattering pseudo-spherical code of Clancy et al., (2017), we estimate that the intensity would have been 1.9 kR if viewed on the limb from orbit.
MastCam-Z imaged the sky using filter position 0 in both cameras starting at 00:43 LMST. Although sky brightness was dominated by Phobos-illuminated aerosols leading to an overall yellow-orange color, after modeling and subtracting the Phobos light we found an excess green-channel signal equivalent to 100 +/- 20 R, fully consistent with the green line emission detected by SuperCam.
Figure 2: SuperCam spectra and best-fit model. Top panel: the average of 2x75 spectra shown in black, and best-fit model shown in green. Bottom panel: residual shown in red.
Summary and discussion
A green atmospheric emission was observed by the M2020 rover on March 18th 2024. The signal observed by SuperCam and MastCam-Z corresponds to an intensity of around 93 R. The observed event was likely associated with a C4.9 flare and a subsequent ICME. Considering that the particle flux increase observed in orbit coincided with the estimated arrival of the ICME, we conclude the observed diffuse aurora was mainly induced by ICME-accelerated SEPs.
This detection confirms the prediction from UV observations that emissions would also occur at optical wavelengths. This opens a new avenue for the study of space weather events at Mars. Optical instruments are simpler and cheaper than the UV instrumentation used to date. While the brightness of this event was dimmed by dust, events under better viewing conditions or heavier particle precipitation would likely be above the threshold for human vision and visible to future astronauts.
Acknowledgements
We thank Olivier Witasse for providing EDAC data, and NASA’s Mars Exploration Program for support. We acknowledge the Community Coordinated Modeling Center at Goddard Space Flight Center for the use of the DONKI tool, https://kauai.ccmc.gsfc.nasa.gov/DONKI/, and the Mars Space Weather Alert Notification system which notified us of the SEP event.
How to cite: Knutsen, E. W., McConnochie, T. H., Lemmon, M., Tamppari, L., Viet, S., Cousin, A., Wiens, R. C., Francis, R., Donaldson, C., Lasue, J., Forni, O., Patel, P., Schneider, N., Carrasco, D. T., and Palacio, V.: First detection of visible-wavelength aurora on Mars, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-767, https://doi.org/10.5194/epsc2024-767, 2024.
Fig. 1: Topside ionospheres of Venus and Mars as seen by (a) VEX VeRa, (b) MEX MaRS and (c) MAVEN ROSE.radio science.
While the orbital and environmental parameters of Venus and Mars show significant differences, their planetary ionospheres show notable similarities (Figure 1). The photochemically dominated regions of the undisturbed dayside ionospheres of Venus and Mars are both characterized by two major features. The ionospheric main peak region (V2 at Venus, M2 at Mars) is a result from photoionization by solar EUV irradiation. The weaker secondary V1/M1 region originates from the primary and secondary ionization of the neutral atmosphere caused by solar X-ray radiation. The upper region of the Venus and Mars dayside ionospheres is governed by transport processes.
The extent and shape of the ionospheric topsides is observed with the radio science experiments Venus Express Radio Science (VeRa) onboard Venus Express (VEX) [1] at Venus and Mars Express Radio Science (MaRS) [2] onboard Mars Express (MEX) and the Radio Occultation Science Experiment (ROSE) onboard the MAVEN spacecraft [3] at Mars. The observed electron density exhibits substantial variability on temporal scales and ranges from an undisturbed exponential decay (Fig. 1b) to strongly compressed shapes (Fig. 1a, c).
This work combines 9 years of VEX-VeRa (2006-2014), 18 years of MEX-MaRS (2004-2021) and 8 years of MAVEN-ROSE (2014-2021) radio occultation observations to investigate the variability of the topside ionospheres of Venus and Mars on the planetary dayside. The derived ionospheric characteristics will be compared to accompanying observations of the solar wind dynamic pressure (from VEX-ASPERA4 [4], MEX-ASPERA3 [5, 6] and MAVEN instruments [7]), solar irradiation flux (FISM-V2 model [8], MAVEN EUV monitor [9]) and a model of the crustal magnetic field for Mars [10] to improve our understanding of the solar wind interaction of planets without a global magnetic field.
References
[1] Häusler et al. (2006) PSS 54 (13-14)
[2] Pätzold et al. (2004) in Fletcher (2004) Mars Express. The scientific payload. ESA
[3] Withers et al. (2018) JGR Space Phys. 123 (5)
[4] Barabash et al. (2007) PSS 55 (12)
[5] Barabash et al. (2006) SSR 126
[6] Ramstad et al. (2015) JGR Planets 120 (7)
[7] Halekas et al. (2015) SSR 195
[8] Chamberlin et al. (2020) Space Weather 18 (12)
[9] Thiemann et al. (2017) JGR Space Physics 122 (3)
[10] Morschhauser et al. (2014) JGR Planets 119 (6)
How to cite: Peter, K., Pätzold, M., Withers, P., Ramstad, R., Thiemann, E., Fränz, M., Häusler, B., Futaana, Y., and Holmström, M.: Dynamics of the topside ionospheres of Venus and Mars as seen by recent radio science observations, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-481, https://doi.org/10.5194/epsc2024-481, 2024.
Introduction
Mars’ lack of a global magnetic field led to low expectations for auroral phenomena on the planet, but MAVEN observations showed auroral activity to be frequent, diverse in nature, and often global in scope. Subsequent observations by the Emirates Mars Mission [1] further expanded the breadth of observable types of aurora, and it is likely that the variety will increase further as observations expand. Figure 1 below shows three fundamentally different types of aurora on Mars. Ironically, Mars’ lack of a global field is actually responsible for most of the activity, which leads to a new perspective for non-magnetized objects in our solar system and beyond.
Making Sense of Diverse Auroral Activity
Each of type of Mars aurora is a tracer for a different important process involving the interaction between solar influences and the near-Mars magnetic and charged particle environment. Originally, types of aurora were named for their resemblance to terrestrial or other types, with later forms named after their morphology, geographical location, precipitating particles, etc. The nomenclature became cumbersome, confusing, and not insightful to those not working in the field. At the same time, modeling showed that all types could fit into three categories based on the precipitating particles and their origins: solar energetic particles (SEPs), suprathermal electrons, and solar wind protons (see Figure 1). These distinctions usefully group the phenomena into clusters of phenomena observed and modelled similarly, and also allow for new varieties to be added.
Figure 1. Three types of aurora on Mars, as observed by the Imaging UltraViolet Spectrograph (IUVS) on MAVEN (left and right images) and the Emirates Ultraviolet Spectrometer (EMUS) on the Emirates Mars Mission (center image). Each is diagnostic of a specific interaction between solar or internal influences and Mars’ magnetic and plasma environment.
SEP Aurora on the Rise to Solar Max
This presentation will focus on SEP aurora, the type of aurora most dependent on solar activity. MAVEN/IUVS discovered that the entire visible nightside of Mars can be engulfed in auroral emissions [Figure 1, left, and reference 2]. The phenomenon can also be studied in limb scan mode, which revealed that
solar energetic particles can penetrate down to ~60 km altitude. Contemporaneous MAVEN/SEP observations of electrons up to 200 keV confirmed the correlation with solar activity. Some diffuse aurora events have been observed to last for days during extended solar events. Both SEP protons and electrons can be responsible [3].
SEP aurora events are currently occurring at the highest frequency of MAVEN’s 10 year mission. The phenomenon was discovered in 2014 during the declining phase of the solar cycle, and only two additional major events occurred in the following 7 years. Since August 2022, IUVS has observed 8 major events, half of which have occurred since February 2024 (Figure 2). (Additional SEP activity did occur throughout this period, but observations at those other times were not possible.) All auroral events are closely correlated with the arrival of SEP particles as measured by MAVEN’s SEP instrument.
Figure 2. MAVEN/IUVS image of SEP aurora (also known as diffuse aurora) on 18 March 2024 during a space weather event. Both the bright limb and on-disk emission are attributed to auroral emission. The near-UV image of Mars’ south polar region in southern spring was obtained contemporaneously with the auroral portion of the image by alternating detector gain during nadir scanning.
The substantial dataset of recent events will be ideal for comparative studies to determine (1) whether protons or electrons are responsible, and which energy ranges matter most; (2) how Mars’ hybrid magnetosphere responds to the SEP flux, and whether the auroral brightness is modulated by local structures or crustal magnetic fields.
In additional to auroral science goals, our efforts aim to quantify how aurora can serve as a proxy for space weather hazard for human exploration of Mars. This leads to the ironic situation where the aurora may be so impressive it’s necessary to seek shelter against the radiation.
Reaching Solar Max does not imply that auroral activity will soon decrease. In fact, the type of solar activity giving rise to SEP events may increase in the declining phase, with more burst of SEP aurora. MAVEN’s potential mission extension will be able to test this hypothesis.
The Case for Visible Nightside Imaging from Orbit.
Visible wavelength emissions are predicted to accompany UV emissions from SEP aurora, based on atomic and molecular physics with known branching ratios [4]. Oxygen green line emission is expected at 557 nm, as frequently seen at Earth. Brightnesses are expected to be detectable with visible wavelength cameras capable of long exposures. No orbital instruments on existing spacecraft have yet made detections, probably due to sensitivity limits in short exposures.
Future Mars missions with instruments designed for nightside imaging offer tremendous low-cost potential for breakthrough observations in auroral science. Visible filter imaging of the aurora with conventional technology is likely to be orders of magnitude more sensitive than the complex and expensive slit-scanning spectral imagers in orbit at Mars today. In addition to the [OI] green line emission, SEP aurora should also cause emission near the blue end of the visible range, emanating from the FDB bands of CO2+. The recent discovery of visible-wavelength nightglow [5] offers another compelling target for nightside imaging. The M-MATISSE mission, currently in a competitive Phase A Study for an ESA M-class mission, carries such a camera [6] . Even if selected, missions beyond should also carry low-cost visible imagers optimized for the orbit, observational capabilities and science goals of the mission.
References: [1] Lillis et al. (2022), GRL, doi: 10.1029/2022GL099820; [2] Schneider, et al. (2018). GRL, doi: 10.1029/ 2018GL077772. [3] Nakamura et al. (2020), JGR, doi: 10.1029/2021JA029914; [4] Gérard et al., (2015) JGR. 120, 6749–6765 doi: 10.1002/ 2015JA021150; [5] Gerard et al., Nat. Astr., 10.1038/s41550-023-02104-8; [6] Sanchez-Cano et al. (2023): doi 10.3389/fspas.2022.1101945
How to cite: Schneider, N., Jain, S., Cessna, J., Connour, K., Deighan, J., Jolitz, R., Lee, C., Larson, D., Rahmati, A., and Curry, S.: Mars Aurora on the Rise to Solar Max, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-581, https://doi.org/10.5194/epsc2024-581, 2024.