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Mem. S.A.It. Vol. 74, 729 c

° SAIt 2003 Memoriedella

The day the solar wind almost disappeared:

a preliminary analysis of the geomagnetic response in the recovery phase.

P. Di Giuseppe and U. Villante

Dipartimento di Fisica, Universit`a and Area di Ricerca in Astrogeofisica. L’Aquila (Italy). e-mail: paolo.digiuseppe@aquila.infn.it

Abstract. On May 10–12, 1999 a prolonged interval of extremely low solar wind density led to a huge distension of the Earth’s magnetosphere. We discuss some aspects of the geomagnetic response observed on May 12, i.e. in the recovery phase of the SW density. The experimental observations confirm a strong morn- ing/afternoon asymmetry in ground observations, and reveal sharp differences in the geomagnetic response in different sectors of the prenoon quadrant. These results suggest that ionospheric contributions might extend to a portion of the prenoon sector larger than predicted.

Key words. SI – Geomagnetic response

1. Introduction

As extensively discussed in a number of pa- pers (Farrugia et al., 2000; Jordanova et al., 2001; Le et al., 2000; Fairfield et al., 2001), on May 10–12, 1999 a prolonged interval of extremely low solar wind (SW) density led to a huge distension of the Earth’s mag- netosphere. In the present paper we con- duct a preliminary analysis of the geomag- netic response to the variable SW condi- tions observed on May 12, i.e. in the recov- ery phase of the SW density. In particular we focused attention on a Sudden Impulse (SI, ∼1550 UT) related with a sharp SW pressure variation observed by Wind space-

Send offprint requests to: P. Di Giuseppe Correspondence to: via Vetoio, 67010 Coppito (AQ)

craft at ∼1537 UT, at a radial geocentric distance of ∼ 56 RT. A more definite dis- cussion of this event will be presented in a forthcoming paper.

2. An analysis of the geomagnetic response.

We examined ground observations in the Northern hemisphere at geomagnetic lati- tudes ranging between 23– 64. As shown in figure 1, the impact of the SW pressure pulse provides a remarkable geomagnetic field variation at each station. However, a simple visual inspection of figure 1 re- veals explicit differences in the experimen- tal observations at different sites which, in our opinion, are more significant than in other cases (Araki, 1994). According

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730 P. Di Giuseppe and U. Villante: LT asymmetry of the geomagnetic response

0 4 8 12 16 20 LT

20°

30°

40°

50°

60°

Λ

H 20 nT

A

A B

C1

C2

0 4 8 12 16 20 LT

20°

30°

40°

50°

60°

Λ

D 20 nT

A

A B

C1

C2

Fig. 1. Ground magnetic field observations (1 min) for the time interval 1545–1615 UT.

H component (North/South) is in the upper panel and D component (East/West) in the lower panel.

to morphological aspects, these observa- tions can be tentatively organized in sev- eral groups, although the physical charac- teristics of events progressively evolve from one to another group.

Group A. Basically, in the nighttime hours (21–2 LT), the geomagnetic field be- havior is mostly characterized by an ex- plicit variation of the H component which identifies the SI occurrence. In this case the D component only shows minor amplitude fluctuations.

Group B. In the afternoon sector (16–

19 LT), the behavior of both components is more structured and explicit variations are observed also in the D component. For

this group of events there is a clear evidence for a positive enhancement in the H com- ponent preceding the SI. This component also shows peak overshoot values.

The results obtained in the prenoon quadrant appear strongly dependent upon latitude and LT.

Group C1. These events reveal, in gen- eral, a minor amplitude geomagnetic re- sponse of the whole pattern. Peak over- shoot values are now observed in the D component, while the H component shows almost constant values after the main vari- ation.

Group C2. For this group of events, the D component reveals approximately

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P. Di Giuseppe and U. Villante: LT asymmetry of the geomagnetic response 731

30° 40° 50°

0 10 20 30

(nT)

ρ = 0.97

Λ H

30° 40° 50°

−30

−20

−10 0

Λ

(nT)

ρ = −0.92 D

LT 1634 − 1700

30° 40° 50°

0 10 20 30

(nT)

Λ ρ = 0.98

H

30° 40° 50°

−30

−20

−10 0

(nT)

Λ ρ = −0.96

D

LT 1806 − 1835

Fig. 2. The asymptotic variation observed in narrow LT strips in the afternoon sec- tor for the H and D component (ρ is the correlation coefficient with latitude).

the same behavior as in group C1, al- though with greater amplitude variations.

Conversely, the H component shows strik- ing differences with respect to C1 events and reveals clear evidence for a positive then negative variation.

In order to compare the present results with those provided by previous investi- gations, we found interesting to estimate the average latitudinal dependence of the asymptotic variation in narrow LT strips in the afternoon sector, where it more explic- itly emerges: the experimental results (fig- ure 2) show a strong correlation between the asymptotic variation and the geomag- netic latitude, and suggest an average lat- itudinal rate for the H component of 0.9 nT/ in the 1634–1700 LT quadrant, and 0.7 nT/in the 1806–1835 LT quadrant; in the same sectors the D component shows gentler latitudinal gradients, such as –0.3 nT/ and –0.5 nT/, respectively.

The global polarization pattern of the main field variation (figure 3) confirms the major importance of the H and D compo- nent in the night and morning sector, re- spectively. As can be seen, a tendency for an elliptical clockwise (CW) polarization (as seen from above the Earth surface) can

be detected in the afternoon sector, where it becomes more clear at higher latitudes after 17 LT.

3. Summary and discussion.

We examined some preliminary aspects of the geomagnetic response to SW pressure variations which were observed on May 12, 1999, i.e. in the recovery phase after the day the SW almost disappeared.

The results of our analysis confirm a strong morning/afternoon asymmetry in the geomagnetic response (Russell and Ginskey, 1995, Francia et al., 1999, 2001).

Several aspects of the present investigation find correspondence in the current under- standing of the SI manifestation (Araki, 1994). They assume, however, greater rele- vance than in previous investigations and reveal clear differences between different sectors in the prenoon quadrant which sug- gest that ionospheric contributions play a major role also at low latitudes in par- ticular in the morning sector. Our results also confirm the major important of the D component for a definite evaluation of several aspects of the geomagnetic response (Tsunomura, 1998).

According to early investigations (Wilson and Sugiura, 1961), from low to high latitudes in the Northern hemisphere, the polarization of the SI field is often elliptical and the polarization pattern is CCW, between 22–10 LT, and CW be- tween 10–22 LT. However, this conclusion was criticized by Matsushita (1962, see also Araki and Allen, 1982), who reported that only two fifths of analyzed SIs indicated elliptical polarization, and only half of those agree with the proposed pattern. In the present case the polarization pattern is mostly linear, and some evidence for an elliptical CW polarization is observed only in the 16–18 LT sector, at higher latitudes.

In this sense, it is interesting to remark that our results can find correspondence in those recently obtained (although in the micropulsation band) at low latitudes by Vellante et al. (2001). They conducted,

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732 P. Di Giuseppe and U. Villante: LT asymmetry of the geomagnetic response

0 4 8 12 16 20 LT

20°

30°

40°

50°

60°

Λ

40 nT 40 nT

Fig. 3. The polarization pattern at different ground stations.

indeed, a statistical analysis of the prop- agation characteristics of ULF waves and found that, while for the entire day a linear polarization was strongly dominant, some evidence for an elliptical polarization was identified only around sunrise and sunset times. In addition, for selected polarized events (10–40 mHz), a dominant CW polarization was observed only between 16–20 LT.

Acknowledgements. The ground data are from the CD-ROM of INTERMAGNET “Magnetic Observatory Definitive Data”, 1999. This re- search activity is supported by MIUR, ASI and Area di Astrogeofisica.

References

Araki, T. and Allen, J. H.: 1982, J.

Geophys. Res. 87, 5207.

Araki, T.: 1994, Geophys Monogr. Ser., 81, 183.

Fairfield, D. H., et al. 2001, J. Geophys.

Res. 106, 25361.

Farrugia, C. J. et al. 2000, Geophys. Res.

Lett. 27, 3773.

Francia, P. et al. 1999, J. Geophys. Res.

104, 19923.

Francia, P., Lepidi, S., Di Giuseppe, P. and Villante, U.: 2001, J. Geophys. Res. 106, 21231.

Jordanova, V. K., Farrugia, C. J., Fennell, J. F. and Scudder, J. D.: 2001, J.

Geophys. Res. 106, 25529.

Le, G., Russell, C. T. and Petrinec, S. M.:

2000, Geophys. Res. Lett. 27, 1827.

Matsushita, S.: 1962, J. Geophys. Res. 67, 3753.

Russell, C. T. and Ginskey, M.: 1995, J.

Geophys. Res. 100, 23695.

Tsunomura, S.: 1998, Earth Planets Space 50, 755.

Vellante, M., Villante, U., Santarelli, L. and De Lauretis, M.: 2001, Solar cycle and space conference proceedings, ESA SP- 477.

Wilson, C. R. and Sugiura, M.: 1961, J.

Geophys. Res. 66, 4097.

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