Österreichische Akademie der Wissenschaften (ÖAW) / Institut für Weltraumforschung (IWF), 8042 Graz, Austria, www.iwf.oeaw.ac.at, Contact: georg.fischer@oeaw.ac.at

The Great White Spot event SED activity after the GWS

Seasonality of SED storms

References

The demise of the GWS

Thunderstorm and lightning observations during and after the GWS event on Saturn

The giant thunderstorm called Great White Spot (GWS) emitted radio waves associated with Saturn lightning (SEDs=Saturn Electrostatic discharges) from 5 December 2010 until 28 August 2011. It was the first SED storm located in the northern hemisphere in the Cassini era [Fischer et al., 2011]. The storm’s head at 35º north was the main center of lightning activity. It grew to a size of 10,000 km in latitude within 3 weeks and developed an elongated eastward tail which wrapped around the whole planet (a distance of 300,000 km) by February 2011. The visible plume (see Figure 1) consists of high-altitude clouds that overshot the outermost ammonia cloud layer due to strong vertical convection, as is typical for thunderstorms. The total power of the storm was comparable to Saturn’s total emitted power of ~1017 W. Previous Great White Spots occurred in 1876, 1903, 1933, 1960, and 1990; i.e. usually once per Saturn year.

Figure 2: Dynamic spectrum (radio wave intensity as a function of time and frequency) measured by the Cassini RPWS (Radio and Plasma Wave Science) instrument on 2 January 2011 during the GWS event. The high SED rate from 21:00 to 03:00 SCET is due to lightning activity in the GWS head region. The low SED rate before comes from a smaller white spot (storm J2) located at a kronocentric latitude of 50º north (see Figure 5).

The lightning activity after the GWS event showed a few episodes around September/October (storm J’), which probably originated from thunderstorm cells located in the former tail of the GWS at 35º north latitude. An SED polarization analysis points to a source in the northern hemisphere. Minor SED activity was also present at the end of Dec. 2011. A 2000-km sized lightning storm developed at the end of March and lasted until 21 April 2012 (storm K). Ground-based images (e.g. Figure 5) showed an associated white spot at a latitude of 50º north. This latitude coincides with a minimum in zonal wind speed in Saturn's troposphere, which is similar to previous SED storms at ±35º. It is very likely that the spot at 50º north was already an active lightning source in January 2011 during the intense GWS event (see Figures 2 & 5), and maybe it showed another small outbreak in July 2012 (storm L). A summary of the SED activity since 2010 is given below in Figure 4 and Table 1.

Figure 6: Latitude of the sun (sinusoidal curve) and of Saturn storms from 1980 until now. The duration and size of the storms are indicated by the width and height of the black rectangles. The question marks indicate that either the location or the convective nature of the storm (for 1994) is uncertain.

Lightning flash rates were extremely high (~10 per second) from mid-December until 20 June. The collision between the anticyclonic vortex with the head of the storm [Sánchez-Lavega et al., 2012; Sayanagi et al., 2012] led to a steep decline in flash rate. After that the SED rate waxed and waned until the SED signals became undetectable in early August. They resurged in late August for 9 days on a weak level, and the source clouds could be identified in the Cassini images.

Georg Fischer1, Kunio M. Sayanagi2, Donald A. Gurnett3, William S. Kurth3, Ulyana A. Dyudina4, Andrew P. Ingersoll4, Marc Delcroix5, Anthony Wesley6, and Trevor Barry7,1 Space Research Institute, Austrian Academy of Sciences, Graz, Austria 2 Atmospheric and Planetary Sciences, Hampton Univ., Hampton, VA, USA

3 Dept. of Physics and Astronomy, The Univ. of Iowa, Iowa City, IA, USA 4 Division of Geological and Planetary Sciences, Caltech, Pasadena, CA, USA

5 Societe Astronomique de France, Tournefeuille, France 6 Canberra Astronomical Society, Murrumbateman, Australia 7 Astronomical Society of New South Wales, Broken Hill, Australia

Figure 3: SED rate per hour as a function of time from mid-June to early Sept. 2011. Panel (a) displays the rate as measured, and panel (c) shows a distance-normalized rate. The distance of Cassini to Saturn’s center in Saturn radii is in panel (b).

Figure 4: Estimated SED rate per Saturn rotation from early 2010 until late 2012. See the table below for more information.

SED storm

name

Start date

Start DOY 2004

End date

End DOY 2004

Storm latitude centric

Remarks

I 07 Feb. 2010 2230 26 June 2010 2369 35°S weak SED activity days 2230-2243

08 July 2010 2381 14 July 2010 2387 35°S same storm (gap in SED activity)

IE1 02 Sept. 2010 2437 03 Sept. 2010 2438 ? irregular episodesJ1 05 Dec. 2010 2531 08 Aug. 2011 2777 35°N Great White Spot (GWS)

18 Aug. 2011 2787 28 Aug. 2011 2797 35°N same storm (gap in SED activity)J2 28 Dec. 2010 2554 08 Jan. 2011? 2565 50°N small white spot, maybe SEDs in Feb.

J' 27 Sept. 2011 2827 09 Oct. 2011 2839 GWS tail? aftermath of GWS, north. hemisphere

IE2 26 Dec. 2011 2917 30 Dec. 2011 2921 ? position unknown, no images

K 26 March 2012 3008 21 April 2012 3034 50°N few SEDs 3009-3017

L 15 July 2012 3119 23 July 2012 3127 50°N? longitude consistent with 50°N spot

Table 1: Names, dates, and storm latitudes of SED storms from 2010 until end of 2012, compare with Figure 4.

Figure 5: Ground-based Saturn observations by T. Akutsu on 13 January 2011 (left side) and A. Wesley on 12 April 2012 (right side). In both cases a small white spot can be seen at a kronocentric latitude of 50º north (56º kronographic). This spot might have drifted with 0.3º/day from ~180º in mid-Jan. 2011 to ~315º west long. in mid-April 2012 [Delcroix et al., 2012]

Figure 1: Development of the GWS from December 2010 until July 2011 as imaged by the Cassini camera. Each panels shows 20.5º to 38.9º kronocentric latitude, and the width of the figure corresponds to 200º in longitude. The storm’s head, dark ovals, and the anticyclonic vortex are marked by red, yellow, and black triangles [from Sayanagi et al., 2012].

Fischer et al., A giant thunderstorm on Saturn, Nature 475, 75-77, 2011.Delcroix et al., Saturn northern hemisphere’s atmosphere after the 2010/2011 Great White Spot, EPSC abstract 934, 2012.Sánchez-Lavega et al., Ground-based observations of the long-term evolution and death of Saturn’s 2010 Great White Spot, Icarus 220, 561-576, 2012.Sayanagi et al., Dynamics of Saturn’s great storm of 2010-2011 from Cassini ISS and RPWS, Icarus, submitted, 2012.The image by T. Akutsu was taken from www.pvol.ehu.es/.