New rms-based planetary geomagnetic activity indices

Transcription

1 New rms-based planetary geomagnetic activity indices M. Menvielle 1,2, J.-J. Valette 3, Mathieu Pau 3, Ch. Lathuillère 4 1 CNRS/INSU, Université Versailles St-Quentin; LATMOS-IPSL, Guyancourt, France 2 Univ Paris-Sud, Département des Sciences de la Terre, Orsay, France 3 CLS, Toulouse, France 4 Laboratoire de Planétologie, UJF, CNRS, BP 53, 3841 Grenoble cedex, France

2 The geomagnetic indices H envelopes PC AE f( H, D, t) Kp J//-Region1 J//-Region2 K-derived ASY-H am H range Dst SYM-H H weighted average Cross-Tail Current Sheet

3 am indices: the network The am network J // -Region1 J // -Region2 Cross-Tail Current Sheet At subauroral latitudes, the irregular variations are not under the predominance of only one system of currents

4 am indices: physical interpretation Experimental estimation Observed and Computed am-indices K : code characterising the magnetic energy density at the station where it is measured (Menvielle, 1979). am : estimate of the energy status of the magnetosphere over the 3-hour interval. Estimation by a semi-empiric equation (Svalgaard, 1978) SW dynamical Pressure Flux of reconnected field lines Geometric effect 2 BV nv am = 6,6 q ( f, α ) ,157 (1 + 3 cos ψ ) 2 2 3

5 The 3-hour interval: a limitation 23, May 29 th to June 1 st DoY PC (nt) 5 am & α15 (nt) AU & AL (nt) ASY-H (nt) SYM-H (nt)

6 Another proxy for the energy Magnetic energy density Modulus of the horizontal components of the K-variations Unit: nt ( B x2 + B y2 ) ½ averaged over a time span of duration τ α τ ( S, t) = i t+ τ t ( 2 2 B ( S, t) + B ( S, t) ) x i y i 1 2 dt

7 Computation of the K-variations The daily S R variations and the K variations Bx(t) = X(t) S R (X,t) Bx(t) = X(t) S R (X,t) are computed using the FMI method.

8 Local index PAF: KERGUELEN BOX : BOROK α 3 (nt) UT Variations observed at two magnetic conjugate auroral stations (24, Nov. 8 th to Nov. 1 th ) UT

9 2 nt X 2 nt Y 2 nt X X Y Y , May 29 - Fredericksburg , May 29 - Hartland , May 29 - Memabetsu UT variability, in response to the solar wind variability. Longitude/LT variability, resulting from the magnetosphere dissymmetry 2 nt 2 nt 2 nt X X Y X Y Y , May 29 - Trelew , May 29 - Hermanus , May 29 - Canberra

10 αm Planetary index: derivation scheme The ideal network would be the am network. At each station, the α τ (Si,t) quantity is computed, and normalized to account for differences in latitude between the stations. The stations are gathered in longitude sectors. For each longitude sector, the normalized α τ (Si,t) quantity are averaged out. Hemispheric indices are the weighted averages of the α τ (Si,t) quantity for the longitude sectors; the weighting coefficients account for the differences in longitude extent of the sectors. The resulting index is an estimate of the mean activity at 5 of corrected magnetic latitude during the [t, t+τ] time interval. αa The planetary index is defined as the average of t+ τ 2 2 α τ (HAD,t) and α τ (CAN,t). ( S, t) = B ( S, t) + B ( S, t) α τ i t ( ) x i y i 1 2 dt

11 K-derived and rms planetary indices: time series Planetary indices: αa and am (nt) αa(τ=18 min.) am Julian day of year 23 Comparison between am and αa(τ=18 min.) during a Bartels rotation

12 K-derived and rms planetary indices: statistics αa(18) indices (3K units) rms (nt) Definitive aa (3K units) range (nt) As a first approximation, aa and αa(τ=18 min.) are proportional: αa(18) / aa ~.4 but the relation between aa and aa is not the same for low and high magnetic activity, as it is the case between the range and the rms.

13 The May 23 superstorm 23, May 29 th to June 1 st DoY PC (nt) 5 am & α15 (nt) AU & AL (nt) ASY-H (nt) SYM-H (nt)

14 The May 23 superstorm 3-hour indices 3-hour indices (nt) DoY 23

15 The May 23 superstorm α3 indices α3 indices (nt) DoY 23

16 Thermosphere forcing by the magnetic activity Most of the thermosphere heating comes from Joule deposit in the auroral zone Heat transfer to equatorial latitudes takes several hours The impact of magnetic activity on thermosphere temperature depends on the distance d to the auroral oval Magnetic activity depends on both UT and longitude (from McPherron, 28)

17 Thermosphere forcing by magnetic activity High solar activity Disturbance coefficients are binned as a function of the previous 3-hour geomagnetic indices. Low solar activity Kp=6 Day time: 1h < LT<18h Night time: 22h< LT< 6h Differences between day- and night-time disappear when local indices are used Results from year 24 published in Lathuillère et al. JGR 28

18 Phase of the thermospheric disturbances Day-time Night-time Disturbances occurs often slightly later on the night side than on the day side of the earth. It occurs also later in the dusk sector than in the dawn one. These phase differences can not be described with 3-hour indices.

19 Low- and mid-latitudes thermosphere response to magnetic activity Correlations between α3 geomagnetic proxies and observed CHAMP or modelled thermosphere response for day-time (1-18 LT) and night-time (22-6 LT). The 3 minutes intervals are defined with respect to the CHAMP orbit (from Lathuillère and Menvielle, 29). During very strong storms (am > ~16 nt), the use of α3 planetary proxies has allowed pointing out a 2-h difference between day-time and night-time response of the low- to mid-latitude thermosphere to magnetic activity that is not predicted by the models.

20 Conclusion We propose a new proxy of the magnetic energy that makes it possible to monitor the geomagnetic activity at a station without constraint on the length of the used time interval. Using this proxy and a derivation scheme similar to that of am indices, we derive geomagnetic indices that monitor the planetary geomagnetic activity at subauroral latitudes without constraint on the time interval. Local and longitude sector indices can also be derived. α15 indices have already been calculated for the years They are available on request, but should be considered as research tools. Quick-look values of α15 indices will be routinely made available at the ISGI Internet homepage with a 2 to 3 minutes delay (before summer 211). The evaluation of the α indices is the next step.

21 Thank you for attention