U. Villante

In-situ observations of the latitudinal gradients of the solar wind parameters during 1976 and 1977

Interplanetary observations from Helios 1, Helios 2, and IMP-8 spacecraft during 1976 and 1977, namely the early portion of solar cycle 21, have been used to investigate the latitudinal gradients of the solar wind parameters with respect to the angular displacement from the current sheet inferred from synoptic HAO white-light maps of the solar corona at 1.75 solar radii. A latitudinal belt of ±25 deg around the current sheet has been investigated. Large gradients for solar wind flow speed, proton density and temperature have been found. Smoother gradients were also found for particle flux, kinetic, gravitational and thermal energy density flux. All these gradients revealed to become smoother going towards the solar cycles maximum. Neither latitudinal nor temporal variations were identified for magnetic and thermal energy density. A remarkable result of this study is that the momentum flux density and the total energy flux density which other authors found to be independent of any longitudinal stream structure were also found to be independent of any latitudinal structure. Moreover, these two parameters did not show any temporal variation during the period of interest.

Geomagnetic response at low latitude to continuous solar wind pressure variations during northward interplanetary magnetic field

A study of the geomagnetic field response to a series of continuous solar wind pressure variations has been performed at a low-latitude station during interplanetary events characterized by stable northward magnetic field conditions. The geomagnetic field H component responds well to the solar wind pressure changes on a timescale of few minutes. The amplitude of the geomagnetic field response is found to depend on local time, showing greater values around local noon and midnight.

A statistical study of MHD discontinuities in the inner solar system: Helios 1 and 2

A statistical study is made of the magnetic field directional discontinuities observed in early 1976 onboard Helios 1 and 2. Strong day-to-day variations of occurrence rates are found on either tangential (TD) or rotational (RD) discontinuities. No large variation (if any) is found versus either heliocentric distance or heliographic latitude. This contradicts previous findings obtained by the same technique on Pioneer 8 data in 1968–69; however, reasons are given to expect different results, under different solar conditions. The most interesting results come from the study of the morphology of discontinuities: first of all, the orientation of TDs and RDs normals (identified by a minimum variance technique) are strongly organized by the average magnetic field, following their progressive directional change when approaching the Sun. The inclination (θn) and azimuthal (ϕn) distributions are gaussian and strongly peaked along the field lines for RDs; as regards TDs the normals are perpendicular to the average field and follow its progressive variation; the θndistribution is isotropic in solid angle, which is interpreted as evidence of crossing of flux tubes on the order of one/hour. Implications of this interpretation in contrast with a turbulent approach are also discussed.

Geomagnetic field variations at low and high latitude during the January 10–11, 1997 magnetic cloud

On Jan. 10–11, 1997 a wide magnetic cloud reached the Earth triggering intense geomagnetic activity. Observations performed at low and very high latitude show that the same features appear simultaneously in correspondence to different changes in the solar wind conditions. In particular, highly polarized modes are simultaneously observed at the same discrete frequencies after the passage of the high density solar wind region following the cloud. SIs and ULF waves polarization are also examined in a wide latitudinal and longitudinal extent.

Observations of bow shock motion during times of variable solar wind conditions

We examine seven periods during which IMP 8 made multiple crossings of Earths bow shock during times when IMP 7 data were available to monitor external solar wind conditions. The positions of the bow shock encounters are consistent with reference shock shape models normalized to the solar wind conditions. We find that multiple crossings can usually be interpreted as being due to changes in the external solar wind parameters. We also find that inward motion of the shock is accompanied by large magnetosheath densities just before the shock sweeps across the spacecraft. We perform a chi-square minimization analysis using a limited set of Rankine-Hugoniot conditions across the bow shocks in order to determine their speeds and normals; we find that the shock velocities are generally consistent with the postulated inward and outward bow shock motions. Whether the crossings are observed on the dawnside or the duskside, most of the bow shock structures are quasi-perpendicular due to changes in the external field orientation just upstream of the shock. The orientations of the normals are consistent with a model in which effects of changes in external conditions propagate as shock shape deformations which move downstream from the nose to the flanks.

An extended investigation of Helios 1 and 2 observations: The interplanetary magnetic field between 0.3 and 1 AU

Helios-1 and 2 spacecraft allowed a detailed investigation of the radial dependence of the interplanetary magnetic field components between 0.3 and 1 AU. The behaviour of the radial component Bris in a very good agreement with Parkers model (Br∼ r-2) and the azimuthal component Bφalso shows a radial dependence which is close to theoretical predictions (Bφ∼ r-1). Experimental results for the normal component Bθand for the field magnitude B are consistent with those from previous investigations. The relative amplitude of the directional fluctuations with periods less than 12 hr is essentially independent of heliocentric distance, while their power decreases approximately as r−3 without any appreciable difference between higher and lower velocity regimes.

Geomagnetic sudden impulses at low latitude during northward interplanetary magnetic field conditions

A statistical analysis of geomagnetic sudden impulses has been performed for data acquired at a low-latitude station during northward interplanetary magnetic field conditions. The local time dependence of the ground response, characterized by very low values in the morning with respect to the afternoon and night sectors and by a clear maximum just after local noon, is very similar to that observed at auroral latitudes, suggesting that signatures of auroral ionospheric currents might be observed also at low latitude. The sense of polarization of the sudden impulses shows a clear reversal from counterclockwise to clockwise a few hours after local noon, consistent with the results obtained for low-frequency geomagnetic fluctuations at the same station. In some cases a transient response (overshoot) is clearly observed; its amplitude is greater when the corrected Dst index is more negative, that is, when the ring current intensity is stronger; moreover, it tends to occur within 1-2 hours after the closest interplanetary magnetic field northward turning. These results seem to indicate that at our station overshoots characterize sudden impulses occurring a short time after periods with high magnetospheric activity. Since the local field line is embedded in the plasmasphere, the overshoot might tentatively be interpreted in terms of an overcompression of the plasmasphere which, after periods with high magnetospheric activity, is less dense and more elastic.

Long period geomagnetic field fluctuations at Terra Nova Bay (Antarctica)

A statistical analysis of the power spectra (0.7–5 mHz) of the geomagnetic field components H and D recorded at Terra Nova Bay (Antarctica) during three austral summers close to the maximum of solar activity reveals power enhancements in the H component at ≈ 3.3, 3.9 and 4.5 mHz, which become more evident during daytime intervals. During intervals characterized by higher solar wind speeds these spectral features more clearly emerge and are also accompanied by other enhancements at lower frequencies (≈ 1.2, 1.9 and 2.7 mHz). The observed frequencies are close to the ones detected both at auroral and low latitudes.

Solar flare effect preceding Halloween storm (28 October 2003): Results of a worldwide analysis

[1] On 28 October 2003 an extreme solar flare with significant increases in the EUV and X-ray flux caused increased photoionization effects in the dayside ionosphere and determined a remarkable solar flare effect (SFE) manifestation that preceded Halloween storm. An analysis conducted at 53 stations revealed that the geomagnetic disturbance had greatest amplitude and longest duration at lowest latitudes in the dayside hemisphere, and its characteristics were basically controlled by the zenith angle of the observing stations. The aspects of the SFE onset and initial phase reveal a close correspondence with those of the EUV flux. At equatorial/electrojet latitudes, the SFE manifestation can be mostly interpreted in terms of a significant enhancement of the preflare current system during normal electrojet conditions, with some evidence for a highly confined counter electrojet in the dawn sector. Additional elements, at higher latitudes, might suggest in these regions a more significant role of the X-ray flux and the onset of additional currents below the normal dynamo current region.

Solar activity dependence of geomagnetic field line resonance frequencies at low latitudes

[1] ULF field line resonance frequencies (f R ) of three different magnetic shells (L = 1.61, 1.71, and 1.83) have been monitored during a 4-year period (2001-2004) using a cross-phase analysis of magnetic measurements recorded at the South European Geomagnetic Array (SEGMA). We find that the variations of the daily averages of f R , which reflect changes in plasmaspheric mass density, follow the variations of the daily values of the 10.7-cm solar radio flux F 10.7 with an estimated time delay of 1-2 days. The analysis of selected events indicates that the sensitivity of f R to short-term (27-day) variations in the solar flux is the same as that for long-term (solar cycle related) variations. On the other hand, the results of the overall statistical analysis seem to indicate a lower sensitivity (by a factor of ∼2) of f R to short-term solar flux variations with respect to long-term variations. Geomagnetic activity effects and/or different solar variabilities of EUV and 10.7-cm flux for different timescales are suggested as a possible cause for such a difference. Experimental results are also compared with those provided by a physical-numerical model of the ionosphere-plasmasphere system. Last, we find some evidence for a slight annual variation in f R with an estimated summer/winter ratio of ∼ 1.1-1.2. The inferred corresponding annual variation in the equatorial mass density is in line with previous estimates for the European longitudinal sector as obtained from whistler measurements.