# Presentation: 653808

## On the long term variation in the geomagnetic and solar activitiesMichel Menvielle

- C.E.T.P., 4 Avenue de Neptune, F-94107 Saint Maur Cedex, France, and Univ. Paris Sud, Orsay, France*michel.menvielle@cetp.ipsl.fr*

## The geomagnetic indices

**Dst**

**SYM-H**

**ASY-H**

**AE**

**Kp**

**am**

**K-derived**

******H envelopes**

******H weighted****average**

******H range**

**PC**

***f*****(********H, ********D, t)**

## am indices: physical interpretation

** **Experimental estimation
**         ****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**)**

**am**

**Flux of reconnected field lines**

**Geometric effect**

**SW dynamical Pressure **

## The K-derived aa index

- Estimated on a daily basis

- The K index is the code corresponding to the class in which falls the 3-hour range in the irregular variation. The limits of these classes depend on the station; they are proportional to:
- K     0   1    2    3    4    5      6     7     8      9
- nT 0   5  10  20  40  70  120 200 330  500

- At sub-auroral latitude stations, the mid-class range amplitude aK is a proxy of the magnetic energy density

**aa = ********N****aK****N**** + ********S****aK****S**** **
**Long term systematic errors in aa values may result from:**
**- errors in the ******** determination**
**- errors in the scale factors of the magnetograms**
**- systematic errors in K scaling**

## aa indices and sunspot numbers

- Both aa indices and Sunspot number data series evidence long term variations during the last 140 years.

## Quantities to be used

- Because of both the variability of the geomagnetic activity and the uncertainties in the aa derivation, **long term averages of robust quantities** should be considered when studying long term evolution

## Day-time vs night-time geomagnetic activity

- During very quiet 3-hour intervals (am < 5 nT), magnetic activity is minimum around midnight, while it is maximum around midnight when am > 10 nT

**The magnetic activity between 21 pm and 3 am LT differs from the daily activity**

## Empirical pdf of aa indices during solar cycles

## Empirical pdf of aa indices during solar cycles

## Empirical pdf of aa indices

**The empirical pdf*****s***** observed after solar cycle 17 (1934-1943) **
**are very different from those observed before this solar cycle**

## Quiet and very quiet days

- Daily value Aa<20 nT

- Daily value Aa<13 nT

- solar cycle 12 (1879-1888)

*Histograms of the Aa values (0 < Aa < 40 nT)*

- solar cycle 20 (1965-1975)

*10-year running average of the number of quiet days with a sliding step of one year *

**The yearly numbers of quiet and very quiet days significantly decrease since 1868 **

*(Ouattara et al., in revision)*

## Most likely value of daily Aa

**The most likely value of daily Aa correlates with the maximum value of the Sunspot Number**

## Summary

## Very quiet days and neutral sheet

*Model of dipolar field submitted to the internal pressure of the solar plasma flow (Pneuman and Kopp, 1971)** *

*Average distribution in the ecliptic plane of the solar wind velocity (scale on the left) in 1976 (circles) and 1977 (triangles) as a function of the angular distance (l) in degrees to the neutral sheet (l = 0) (Bruno et al., 1986). Scale on the right: the velocity associated aa index according to Svalgaard (1977). *

**The thinner in average the neutral sheet, the smaller the yearly number of very quiet days.**

- Low solar wind velocity

## Conclusion

**Next steps:**
**- active days**
**- extreme events**

## Thank you for attention