Saturn’s ionosphere:Electron density altitude profiles and ring

shadowing effects from the Cassini Grand Finale.

L. Z. Hadid (1), M. W. Morooka (1), J-E. Wahlund (1), A. M. Persoon (2), D. J. Andrews (1), O.Shebanits (3), W. M. Farrell (4), W. S. Kurth (2), N. J. T. Edberg (1), E. Vigren (1), L. Moore (5), M. M.Heddman (6), T. E. Cravens (7), A. F. Nagy (8), A. I. Eriksson (1)(1) Swedish Institute of Space Physics, Uppsala, Sweden, (2) Department of Physics and Astronomy,University of Iowa, USA, (3) Space and Atmospheric Physics, Imperial College, UK, (4)NASA/Goddard Space Flight Center, Maryland, USA, (5) Center for Space Physics, Boston University,USA, (6) Department of Physics, University of Idaho, USA, (7) Department of Physics and Astronomy,University of Kansas, USA, (8) Climate and Space Sciences and Engineering, University of Michigan,USA. (lina.hadid@irfu.se)

Abstract

Using the latest in-situ measurements of theLangmuir probe (LP) part of the Radio &Plasma Wave Science (RPWS) instrumentpackage onboard the Cassini spacecraft, weanalyse the electron density altitude profilesof all the proximal orbits from the CassiniGrand Finale.Firstly, we study the electron density (ne) al-titude (h) profile by comparing the northernand the southern hemispheres. We constructan average ionospheric profile in the north-ern hemisphere and show a layered structure.Similar layers were observed during the Fi-nal Plunge of Cassini, where the main iono-spheric peak is crossed at ∼ 1550 km alti-tude [1]. Secondly, from the ring shadowsignatures on the total ion current, we repro-duce the A and B ring boundaries and con-firm some of their optical properties. Further-more, observed variable response of the iono-sphere to the B ring shadow implies differentproton production rate and plasma transportfrom the C-D rings [2].

1. Electron density altitudeprofile model

We show in Figure 1-a the consecutive h, ne

profiles of all the proximal orbits measuredby the LP and for some cases estimated fromthe plasma wave frequency characteristicsof the RPWS investigation. The black linerepresents the averaged topside ionosphericprofile up to 5500 km. As one can see, itexhibits a layered structure with distinctregions denoted by P (h > 4000 km),D (2200 km . h . 4000 km) and C(h < 2200 km). Region P, which weinterpret as the inner part of the plasma-sphere, is characterized by near-constantne values with respect to altitude before itstarts to increase around 4500 km. RegionD represents a diffusive equilibrium region,it is highly variable and structured. RegionC, which is a chemical equilibrium layer, ismore stable and regular and dominated byheavy ions [3, 4].

In Figure 1-b, we present the “FinalPlunge” orbit. One can clearly relate the three

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EPSC AbstractsVol. 12, EPSC2018-226, 2018European Planetary Science Congress 2018c© Author(s) 2018

EPSCEuropean Planetary Science Congress

Figure 1: (a) The averaged h, ne profile (darkblue curve) over all the proximal orbits (pur-ple curves) in the northern hemisphere. (b)The h, ne profile of the Final Plunge or-bit. The color code shows the correspondingmagnetic L-Shell values.

different ionospheric regions highlighted inthe averaged profile. The color code corre-sponds to the magnetic L-Shell values andshows that region D maps well inside the in-ner edge of the D ring. This implies thatthe observed irregularities density profiles inlayer D not connected to features in the rings.The red lines represent the estimation of theplasma scale height by fitting with an expo-nential curve for each of the regions P, D andC. Last but not least, during the Final Plungeorbit the LP have revealed a density peak of∼ 1.5×104 cm−3 around 1550 km, which weinterpret as the “main peak” of the Kronianionosphere.

2. A and B ring shadowingeffects

As Saturn’s northern hemisphere experiencedsummer during the Grand Finale, the planet’snorthern dayside hemisphere and its ringswere fully illuminated by the sun. However

the southern hemisphere was partly obscuredbecause of the shadows cast by the A and Brings, which are opaque to the Extreme Ul-traviolet (EUV) solar radiation. As a con-sequence, this caused noticeable decrease inthe amount of ionization in the southern por-tion of the ionosphere which was clearly ob-served by a decrease in the total DC ion cur-rent (IDC,tot) collected by the LP (blue curvein Figure 2).

D C B AB1 B2 B3 B4

Figure 2: Example of one proximal orbit(Rev 276) passing between Saturn (in black)and the rings (green rectangles). The bluecurve represents the total DC current for neg-ative bias voltage projected on the orbit.

As is shown in Figure 2, considering a so-lar elevation angle of 26.3◦, we project theoutbound slant passes (larger for local timesaway from noon) onto the ring plane, andcompare the observed projected boundariesof the shadowed regions (blue dots) with thering boundaries.

In Figure 3 we summarize the distancesfrom Saturn of the observed starting and end-ing points of the shadows compared with theA and B ring boundaries (black dashed lines).The color code represents the total DC cur-rent. Based on the current values, we con-sider the LP in complete shadow when the

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272 274 276 278 280 282 284 286 288 290 292

Proximal orbits

1.4

1.6

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nd

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ing

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Figure 3: Summary of the observed inner andouter edges of the shadows casted by the A(triangle and square respectively) and the Brings (circle and diamonds respectively). Thedashed black lines represents the inner andouter boundaries of the A and B rings.

collected current is below the photoelectronlevel (< 0.7 nA) or around the noise level(0.1 nA). As one can see, the observed edgesof the shadow cast by the A ring (squaresand upward triangles) match very well withits boundaries. Regarding the B ring, the ob-served outer edges of the shadow (Diamonds)match the outer edge of the ring consistentlyfrom one flyby to another. However this isnot the case for the inner edges of the shadow(circles), they are neither uniform nor match-ing the B ring inner boundary. For each orbit,we represent by a downward triangle the dis-tance at which IDC,tot starts to decrease andby a circle the distance at which the LP isshadowed by the B ring (IDC,tot ∼ 0.1 nA).For most the flybys, IDC,tot starts to decreasearound the inner edge of the B ring.The observed variations in the starting pointof the B ring shadow around 4000 km, fordistances outside 1.65 Rs, could be related tothe changes of the proton production rate re-sulting in different responses of the molecularhydrogen to the shadowing effect. Further-

more, plasma transport processes from/to therings could also inhibit the shadowing signa-ture from the B ring.

3. SummaryThis study has provided detailed analysis ofthe Kronien topside ionosphere by evidenc-ing a layered structure with at least a diffusiveand chemical equilibrium regions. Moreover,based on the observed A and B ring shad-ows we could study the reponses of the iono-spheric plasma in the absence of the sunlightas an ionization source.

References[1] Hadid, L. Z., Morooka, M. W., Wahlund, J-

E., Persoon, A. M., Andrews, D. J., Shebanits,O., Kurth, W. S., Vigren, E., Edberg, N. J. T.,Nagy, A. F., and Eriksson, A. I., (2018), Geo-physical Research Letters, submitted.

[2] Hadid, L. Z., Morooka, M. W., Wahlund, J-E., Moore, L, Cravens, T. E., Hedman, M. M.,Edberg, N. J. T, Vigren, E., Kurth, W. S., Far-rell, W. M., and Eriksson, A. I., (2018). Geo-physical Research Letters, submitted.

[3] Morooka, M. W., Wahlund, J.-E., Hadid, L.Z., Edberg, N. J. T., Vigren E., Andrews, D. J.,Persoon, A. M., Kurth, W. S., Gurnett, D. A.,Farrell, W. M. Waite, J. H., Perryman, R. S.,Perry, M., Mitchell, D. G. and Hsu, S. (2018).Geophysical Research Letters, submitted.

[4] Waite, J.H., Jr., R. Perryman, M. Perry, K.Miller, J. Bell, T.E. Cravens, C.R. Glein, J.Grimes, M. Hedman, T. Brockwell, B. Teo-lis, L. Moore, D. Mitchell, A. Persoon, W.S.Kurth, J.-E. Wahlund, M. Morooka, L. Z. Ha-did, S. Chocron, J. Walker, A. Nagy, R. Yelle,S. Ledvina, R. Johnson, W. Tseng, O.J. Tuckerand W.-H. Ip (2018), Science, submitted.

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