Diamagnetic depression observations at Saturn’s magnetospheric cusp by the Cassini spacecraft

The magnetospheric cusp is a region where shocked solar wind plasma can enter a planetary magnetosphere, after magnetic reconnection has occurred at the dayside magnetopause or in the lobes. The dense plasma that enters the high‐latitude magnetosphere creates diamagnetic effects whereby a depression is observed in the magnetic field. We present observations of the cusp events at Saturn’s magnetosphere where these diamagnetic depressions are found. The data are subtracted from a magnetic field model, and the calculated magnetic pressure deficits are compared to the particle pressures. A high plasma pressure layer in the magnetosphere adjacent to the cusp is discovered to also depress the magnetic field, outside of the cusp. This layer is observed to contain energetic He++ (up to ∼100 keV) from the solar wind as well as heavy water group ions (W+) originating from the moon Enceladus. We also find a modest correlation of diamagnetic depression strength to solar wind dynamic pressure and velocity; however, unlike at Earth, there is no correlation found with He++ counts.Key PointsDiamagnetic depressions are found in the cusp and are observed to continue into the adjacent magnetosphereA heated plasma layer of mixed composition is found to depress the adjacent magnetospheric fieldDiamagnetic depression strength is correlated to solar wind dynamic pressure and velocity but not to the observed He++ counts, like at EarthPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137687/1/jgra53517_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137687/2/jgra53517-sup-0001-supinfo.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137687/3/jgra53517.pd

Saturn's open‐closed field line boundary:a Cassini electron survey at Saturn's magnetosphere

We investigate the average configuration and structure of Saturn's magnetosphere in the nightside equatorial and high‐latitude regions. Electron data from the Cassini Plasma Spectrometer's Electron Spectrometer (CAPS‐ELS) is processed to produce a signal‐to‐noise ratio for the entire CAPS‐ELS time of operation at Saturn's magnetosphere. We investigate where the signal‐to‐noise ratio falls below 1, to identify regions in the magnetosphere where there is a significant depletion in the electron content. In the nightside equatorial region we use this to find that the most planetward reconnection x‐line location is at 20 – 25 RS downtail from the planet in the midnight to dawn sector. We also find an equatorial dawn‐dusk asymmetry at a radial distance of >20 RS which may indicate the presence of plasma depleted flux tubes returning to the dayside after reconnection in the tail. Furthermore, we find that the high‐latitude magnetosphere is predominantly in a state of constant plasma depletion and located on open field lines. We map the region of high‐latitude magnetosphere that is depleted of electrons to the polar cap to estimate the size and open flux content within the polar caps. The mean open flux content for the northern and southern polar caps are found to be 25±5 and 32±5 GWb, respectively. The average location of the open‐closed field boundary is found at invariant colatitudes of 12.7±0.6° and 14.5±0.6°. The northern boundary is modulated by planetary period oscillations more than the southern boundary

Flux transfer event observation at Saturn's dayside magnetopause by the Cassini spacecraft

We present the first observation of a flux rope at Saturn's dayside magnetopause. This is an important result because it shows that the Saturnian magnetopause is conducive to multiple X-line reconnection and flux rope generation. Minimum variance analysis shows that the magnetic signature is consistent with a flux rope. The magnetic observations were well fitted to a constant-α force-free flux rope model. The radius and magnetic flux content of the rope are estimated to be 4600–8300 km and 0.2–0.8 MWb, respectively. Cassini also observed five traveling compression regions (remote signatures of flux ropes), in the adjacent magnetosphere. The magnetic flux content is compared to other estimates of flux opening via reconnection at Saturn

Photoionization Loss of Mercury’s Sodium Exosphere: Seasonal Observations by MESSENGER and the THEMIS Telescope

We present the first investigation and quantification of the photoionization loss process to Mercury’s sodium exosphere from spacecraft and ground‐based observations. We analyze plasma and neutral sodium measurements from NASA’s MESSENGER spacecraft and the THEMIS telescope. We find that the sodium ion (Na+) content and therefore the significance of photoionization varies with Mercury’s orbit around the Sun (i.e., true anomaly angle: TAA). Na+ production is affected by the neutral sodium solar‐radiation acceleration loss process. More Na+ was measured on the inbound leg of Mercury’s orbit at 180°–360° TAA because less neutral sodium is lost downtail from radiation acceleration. Calculations using results from observations show that the photoionization loss process removes ∼1024 atoms/s from the sodium exosphere (maxima of 4 × 1024 atoms/s), showing that modeling efforts underestimate this loss process. This is an important result as it shows that photoionization is a significant loss process and larger than loss from radiation acceleration.Plain Language SummaryMercury has a thin sodium collision‐less atmosphere (i.e., an exosphere). A variety of processes add or subtract sodium particles to and from the exosphere. Photoionization is a loss process, and we investigate it in this paper by analyzing data from NASA’s MESSENGER spacecraft and ground‐based observations made by the THEMIS telescope. Mercury has an eccentric (noncircular) orbit, which means the planet’s distance from the Sun changes throughout its orbit. This, first of all, affects how much sodium is lost due to acceleration of neutral sodium by radiation (i.e., how much sodium is accelerated away from Mercury by radiation from the Sun). This subsequently affects how much sodium is left to be photoionized. Therefore, the amount of sodium lost due to photoionization varies throughout a Mercury‐year. We calculate that ∼1024 atoms/s of sodium are lost due to photoionization, and that it is a significant loss process in comparison to acceleration by radiation.Key PointsPhotoionization can be a significant loss process to the sodium exosphere with peak loss estimates of 4 × 1024 atoms/sThe photoionization loss process of Mercury’s sodium exosphere varies throughout the planet’s orbit around the SunMore sodium is lost due to photoionization on the inbound leg (true anomaly angle of 180°–360°) of Mercury’s orbit than the outbound legPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/167426/1/grl62199.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/167426/2/grl62199_am.pd

Flux Transfer Events at a Reconnection‐Suppressed Magnetopause: Cassini Observations at Saturn

We present the discovery of seven new flux transfer events (FTEs) at Saturn’s dayside magnetopause by the Cassini spacecraft and analyze the observations of all eight known FTEs. We investigate how FTEs may differ at Saturn where the magnetopause conditions are likely to diamagnetically suppress magnetic reconnection from occurring. The measured ion- scale FTEs have diameters close to or above the ion inertial length di- ¼1- 27 (median and mean values of 5 and 8), considerably lower than typical FTEs found at Earth. The FTEs magnetic flux contents are 4- 461 kWb (median and mean values of 16 and 77 kWb), considerably smaller (<0.1%) than average flux opened during magnetopause compression events at Saturn. This is in contrast to Earth and Mercury where FTEs contribute significantly to magnetospheric flux transfer. FTEs therefore represent a negligible proportion of the amount of open magnetic flux transferred at Saturn. Due to the likely suppression of the two main growth- mechanisms for FTEs (continuous multiple x- line reconnection and FTE coalescence), we conclude that adiabatic expansion is the likely (if any) candidate to grow the size of FTEs at Saturn. Electron energization is observed inside the FTEs, due to either Fermi acceleration or parallel electric fields. Due to diamagnetic suppression of reconnection at Saturn’s magnetopause, we suggest that the typical size of FTEs at Saturn is most likely very small, and that there may be more di- ¼1 FTEs present in the Cassini magnetometer data that have not been identified due to their brief and unremarkable magnetic signatures.Key PointsEight Saturn ion- scale flux transfer events (FTEs) are analyzed with diameters of di- ¼1- 27FTEs at Saturn are found to transfer negligible amounts of flux at Saturn’s magnetosphereEvidence for electron energization is observed inside some of the FTEs, due to either Fermi acceleration or parallel electric fieldsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/166408/1/jgra56227_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/166408/2/jgra56227.pd

Photoionization Loss of Mercury's Sodium Exosphere: Seasonal Observations by MESSENGER and the THEMIS Telescope

We present the first investigation and quantification of the photoionization loss process to Mercury’s sodium exosphere from spacecraft and ground‐based observations. We analyze plasma and neutral sodium measurements from NASA’s MESSENGER spacecraft and the THEMIS telescope. We find that the sodium ion (Na+) content and therefore the significance of photoionization varies with Mercury’s orbit around the Sun (i.e., true anomaly angle: TAA). Na+ production is affected by the neutral sodium solar‐radiation acceleration loss process. More Na+ was measured on the inbound leg of Mercury’s orbit at 180°–360° TAA because less neutral sodium is lost downtail from radiation acceleration. Calculations using results from observations show that the photoionization loss process removes ∼1024 atoms/s from the sodium exosphere (maxima of 4 × 1024 atoms/s), showing that modeling efforts underestimate this loss process. This is an important result as it shows that photoionization is a significant loss process and larger than loss from radiation acceleration.Plain Language SummaryMercury has a thin sodium collision‐less atmosphere (i.e., an exosphere). A variety of processes add or subtract sodium particles to and from the exosphere. Photoionization is a loss process, and we investigate it in this paper by analyzing data from NASA’s MESSENGER spacecraft and ground‐based observations made by the THEMIS telescope. Mercury has an eccentric (noncircular) orbit, which means the planet’s distance from the Sun changes throughout its orbit. This, first of all, affects how much sodium is lost due to acceleration of neutral sodium by radiation (i.e., how much sodium is accelerated away from Mercury by radiation from the Sun). This subsequently affects how much sodium is left to be photoionized. Therefore, the amount of sodium lost due to photoionization varies throughout a Mercury‐year. We calculate that ∼1024 atoms/s of sodium are lost due to photoionization, and that it is a significant loss process in comparison to acceleration by radiation.Key PointsPhotoionization can be a significant loss process to the sodium exosphere with peak loss estimates of 4 × 1024 atoms/sThe photoionization loss process of Mercury’s sodium exosphere varies throughout the planet’s orbit around the SunMore sodium is lost due to photoionization on the inbound leg (true anomaly angle of 180°–360°) of Mercury’s orbit than the outbound legPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/167426/1/grl62199.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/167426/2/grl62199_am.pd

Flux Transfer Events at a Reconnection- Suppressed Magnetopause: Cassini Observations at Saturn

We present the discovery of seven new flux transfer events (FTEs) at Saturn’s dayside magnetopause by the Cassini spacecraft and analyze the observations of all eight known FTEs. We investigate how FTEs may differ at Saturn where the magnetopause conditions are likely to diamagnetically suppress magnetic reconnection from occurring. The measured ion- scale FTEs have diameters close to or above the ion inertial length di- ¼1- 27 (median and mean values of 5 and 8), considerably lower than typical FTEs found at Earth. The FTEs magnetic flux contents are 4- 461 kWb (median and mean values of 16 and 77 kWb), considerably smaller (<0.1%) than average flux opened during magnetopause compression events at Saturn. This is in contrast to Earth and Mercury where FTEs contribute significantly to magnetospheric flux transfer. FTEs therefore represent a negligible proportion of the amount of open magnetic flux transferred at Saturn. Due to the likely suppression of the two main growth- mechanisms for FTEs (continuous multiple x- line reconnection and FTE coalescence), we conclude that adiabatic expansion is the likely (if any) candidate to grow the size of FTEs at Saturn. Electron energization is observed inside the FTEs, due to either Fermi acceleration or parallel electric fields. Due to diamagnetic suppression of reconnection at Saturn’s magnetopause, we suggest that the typical size of FTEs at Saturn is most likely very small, and that there may be more di- ¼1 FTEs present in the Cassini magnetometer data that have not been identified due to their brief and unremarkable magnetic signatures.Key PointsEight Saturn ion- scale flux transfer events (FTEs) are analyzed with diameters of di- ¼1- 27FTEs at Saturn are found to transfer negligible amounts of flux at Saturn’s magnetosphereEvidence for electron energization is observed inside some of the FTEs, due to either Fermi acceleration or parallel electric fieldsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/166408/1/jgra56227_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/166408/2/jgra56227.pd

Proton Precipitation in Mercury’s Northern Magnetospheric Cusp

Ion precipitation onto Mercury’s surface through its magnetospheric cusps acts as a source of planetary atoms to both Mercury’s exosphere and magnetosphere. Through the process of ion sputtering, solar wind ions (∼95% protons) impact the surface regolith and liberate material, mostly as neutral atoms. We have identified 2760 northern magnetospheric cusp crossings throughout the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) mission, based on enhancements in proton flux observed by the Fast Imaging Plasma Spectrometer (FIPS). We find cusp crossings spanning 50–85° in magnetic latitude with a geometric center typically at 60–70°. The cusp center is stable about its average but its latitudinal extent varies orbit-to-orbit. The mean latitude weakly depends on the magnitude of the interplanetary magnetic field (IMF), dropping by about 1.3° magnetic latitude for each increase of 10 nT in IMF strength. We have used these identified cusp boundaries to estimate the flux of protons which will precipitate onto Mercury’s surface. We find an average proton precipitation flux of 1.0 × 107 cm−2 s−1, ranging 3.3 × 104–6.2 × 108 cm−2 s−1, and that this flux can vary substantially between subsequent 10-s measurements. We also tabulated the peak precipitation fluxes for each cusp crossing. They range 9.8 × 104–1.4 × 109 cm−2 s−1, with a mean of 3.7 × 107 cm−2 s−1. We find strong dependencies on the local time of the cusp crossing as well as on Mercury’s orbit around the Sun, which warrant further investigation.Key PointsMercury’s northern cusp was found at 50°–85° magnetic latitude in 2760 MErcury Surface, Space ENvironment, GEochemistry and Ranging orbits, falling by 1.3° per 10 nT increase in interplanetary magnetic fieldAverage proton precipitation flux was 1.0 × 107 cm−2 s−1, ranging 3.3 × 104–6.2 × 108 cm−2 s−1 in the 707 orbits with B vector in viewProton precipitation flux can vary by two orders of magnitude in subsequent 10-s measurements with peak fluxes up to 1.4 × 109 cm−2 s−1Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/175086/1/jgra57456.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/175086/2/2022JA030397-sup-0001-Supporting_Information_SI-S01.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/175086/3/jgra57456_am.pd

MESSENGER observations of planetary ion enhancements at Mercury's northern magnetospheric cusp during Flux Transfer Event Showers

At Mercury, several processes can release ions and neutrals out of the planet's surface. Here we present enhancements of dayside planetary ions in the solar wind entry layer during flux transfer event (FTE) "showers" near Mercury's northern magnetospheric cusp. The FTE showers correspond to the intervals of intense magnetopause reconnection of Mercury's magnetosphere, which form a solar wind entry layer equatorward of the magnetospheric cusps. In this entry layer, solar wind ions are accelerated and move downward (i.e. planetward) toward the cusps, which sputter upward-moving planetary ions within 1 minute. The precipitation rate is enhanced by an order of magnitude during FTE showers and the neutral density of the exosphere can vary by >10% due to this FTE-driven sputtering. These in situ observations of enhanced planetary ions in the entry layer likely correspond to an escape channel of Mercury's planetary ions, and the large-scale variations of the exosphere observed on minute-timescales by Earth observatories. Comprehensive, future multi-point measurements made by BepiColombo will greatly enhance our understanding of the processes contributing to Mercury's dynamic exosphere and magnetosphere.Comment: 34 pages, 10 figures, 1 tabl

The impact of an ICME on the Jovian X-ray aurora

We report the first Jupiter X-ray observations planned to coincide with an interplanetary coronal mass ejection (ICME). At the predicted ICME arrival time, we observed a factor of ∼8 enhancement in Jupiter's X-ray aurora. Within 1.5 h of this enhancement, intense bursts of non-Io decametric radio emission occurred. Spatial, spectral, and temporal characteristics also varied between ICME arrival and another X-ray observation two days later. Gladstone et al. (2002) discovered the polar X-ray hot spot and found it pulsed with 45 min quasiperiodicity. During the ICME arrival, the hot spot expanded and exhibited two periods: 26 min periodicity from sulfur ions and 12 min periodicity from a mixture of carbon/sulfur and oxygen ions. After the ICME, the dominant period became 42 min. By comparing Vogt et al. (2011) Jovian mapping models with spectral analysis, we found that during ICME arrival at least two distinct ion populations, from Jupiter's dayside, produced the X-ray aurora. Auroras mapping to magnetospheric field lines between 50 and 70 RJ were dominated by emission from precipitating sulfur ions (S7+,…,14+). Emissions mapping to closed field lines between 70 and 120 RJ and to open field lines were generated by a mixture of precipitating oxygen (O7+,8+) and sulfur/carbon ions, possibly implying some solar wind precipitation. We suggest that the best explanation for the X-ray hot spot is pulsed dayside reconnection perturbing magnetospheric downward currents, as proposed by Bunce et al. (2004). The auroral enhancement has different spectral, spatial, and temporal characteristics to the hot spot. By analyzing these characteristics and coincident radio emissions, we propose that the enhancement is driven directly by the ICME through Jovian magnetosphere compression and/or a large-scale dayside reconnection event