Cassini SOI
Cassini/RPWS/HFR-Kronos
Access to Lesia-Meudon data products
contact
HFR Data Access paginated
HFR Data Access
Browse HFR Data
HFR OpMode DB Search
HFR Routines
HFR Documentation
SKR Phase Systems
Auroral Region Passes
QTN dataset
Admin Access
Southern and Northern SKR phase systems
General Description
Data Access
Publications
Community
VO Access
SKR rotational modulation
The Saturn Kilometric Radiation (SKR) is an intense non-thermal radio emission produced in auroral regions by accelerated electrons moving along magnetic field lines, predominantly on the dawn sector (see e.g. the review of [1]). Its regular pulsation, whose origin remains unexplained yet, was originally interpreted as a clock-like rotational modulation triggered by the planetary magnetic field, and thus directly relating to the planetary interior.
SLS models
The Voyager determination of the SKR period (10h39min24±7s or 10.652±0.002h) [2] was adopted as the official rotation rate of the planet by the International Astronomical Union [3], and used to define the first Saturn Longitude System (or SLS 1). In this system, the SKR occurrence or intensity is organized by sub-solar longitudes and peaks at a fixed arbitrary value (historically set at 100°).
Ulysses observations later showed that the observed SKR period (~10.8h) is not constant but varies with time by 1% over years [4]. Further Cassini studies identified short-term 25 days oscillations of the southern SKR period [5] correlated to the fluctuations of the solar wind speed, together with long-term yearly oscillations that were also observed in magnetospheric plasma and magnetic field data and attributed to Enceladus mass-loading [6]. To account for long-term yearly variations of the SKR period, new longitude systems (SLS 2,3), based on polynomial fits of the SKR phase successively replaced SLS 1 [7,8]. SLS 2 and 3 were defined similarly to SLS 1, so that SKR maxima occur about sub-solar longitudes of 100°.
SKR northern/southern periods
The identification of a second SKR period (~10.6h) [8,9], unambiguously confirmed by different techniques [10,11], has strong implications on the validity of the longitude systems described above. Indeed, SKR periods at ~10.8h and ~10.6h were attributed to SKR emissions emanating from southern (S) and northern (N) hemispheres, and vary with time [9].
More recently, we have computed S,N SKR periods separately with a Lomb-Scargle analysis [12] (Figure 1), and associated phase systems (Figure 2), that organize S and N SKR maxima until 2010 (Figure 3). The description of the technique as well as obtained results are described in [13], and related publications are indicated here.
Data Access
This page provides an access to this dataset here, that includes time series of northern/southern SKR periods, phases and sub-solar longitudes between DOY 2004-001 and 2010-193.
Figures
SKR periods
Fig. 1. Lomb-Scargle periodograms of (a) total, (b) southern and (c) northern SKR power radiated between 40 and 500 kHz as a function of period and time. Individual power spectra (columns, in arbitrary units) were computed over a 200-days long sliding window with a time step of 1 day. Solid yellow lines show S and N periods, as derived from panels (b) and (c).
[Large PNG](1.0MB) [High Res PDF](15.2MB)
SKR phases
Fig. 2. [Open with acrobat reader for best resolution] (a) Plot of S SKR power normalized to the average over one rotation as a function of an arbitrary phase computed from a fixed reference period of 10.7928 h (enabling direct comparisons with [7,8]), between 2004-001 and 2010-193. Each rotation is replicated four times along the y axis for clarity. The S SKR rotational modulation (phase = 0°) is displayed in solid yellow, while the SLS-3 one (sub-solar longitude = 100°) is shown in solid light blue. Both correctly track S SKR maxima. (b) Identical plot for N SKR power and an arbitrary reference period of 10.6 h. (c) Cassini orbital parameters.
[Large PNG](791KB) [High Res PDF](44.5MB)
SKR rotational modulations
Fig. 3. Example of SKR dynamic spectra of flux (top) and circular polarization (bottom) for 2 days of near-equatorial observations in 2007. Emissions originating from both hemispheres are mixed. In the bottom panel, southern R-X mode SKR corresponds to V>0 (black) and northern R-X mode SKR to V<0 (white). Timings of SKR peaks are indicated by arrows. Top arrows correspond to SLS3 timings (sub-solar longitude = 100°), whereas bottom arrows correspond to Southern (black) and Northern (white) SKR peak timings (phase = 0°). As S and N SKR periods are different, SKR emission emanating from both hemispheres superimpose and may hide the modulation, when not in phase.
[Large PNG](614KB) [High Res PDF](19.2MB)
References
[1] Kurth et al., 2009. [link]
[2] Desch and Kaiser, 1981. [link]
[3] Seidelman et al, 2002. [link]
[4] Galopeau and Lecacheux, 2000. [link]
[5] Zarka et al, 2007. [link]
[6] Gurnett et al, 2007. [link]
[7] Kurth et al, GRL, 2007. [link]
[8] Kurth et al, JGR, 2008. [link]
[9] Gurnett et al, GRL, 2009. [link]
[10] Lecacheux, RPWS team meeting, 2009. [PDF] [PPT] (restricted access)
[11] Lamy, RPWS team meeting, 2009. [PDF]
[12] Lamy, PRE7, 2011. [PDF]

Valid HTML 4.01 Strict