A. Dal Lago

Magnetic cloud field intensities and solar wind velocities

For the sets of magnetic clouds studied in this work we have shown the existence of a relationship between their peak magnetic field strength and peak velocity values, with a clear tendency that clouds which move at higher speeds also possess higher core magnetic field strengths. This result suggests a possible intrinsic property of magnetic clouds and also implies a geophysical consequence. The relatively low field strengths at low velocities is presumably the cause of the lack of intense storms during low speed ejecta. There is also an indication that this type of behavior is peculiar for magnetic clouds, whereas other types of non cloud-driver gas events do not seem to show a similar relationship, at least for the data studied in this paper. We suggest that a field/speed relationship for magnetic clouds, as that obtained in our present study, could he associated with the cloud release and acceleration mechanism at the sun. Since for magnetic clouds the total field tyically has a substantial southward component, B s , our results imply that the interplanetary dawn-dusk electric field, given by v X B s (where v is the clouds velocity), is enhanced by both factors. Therefore, the consequent magnetospheric energization (that is governed by this electric field) becomes more efficient for the occurrence, of magnetic storms.

Solar and interplanetary causes of very intense geomagnetic storms

Abstract The dominant interplanetary phenomena causing intense magnetic storms are the interplanetary manifestations of fast coronal mass ejections (CMEs). Two interplanetary structures are important for the development of such class of storms, involving an intense and long duration B s component of the IMF: the sheath region just behind the forward shock, and the CME ejecta itself. Frequently, these structures lead to the development of intense storms with two-step growth in their main phases. These structures also lead sometimes to the development of very intense storms, especially when an additional interplanetary shock is found in the sheath plasma of the primary structure accompanying another stream. The second stream can also compress the primary cloud, intensifying the B s field, and bringing with it an additional B s structure. Thus, at times very intense storms are associated with three or more B s structures. We also discuss evidence that magnetic clouds with very intense core magnetic fields tend to have large velocities, thus implying large amplitude interplanetary electric fields that can drive very intense storms.

Drift Effects and the Cosmic Ray Density Gradient in a Solar Rotation Period: First Observation with the Global Muon Detector Network (GMDN)

We present for the first time hourly variations of the spatial density gradient of 50 GeV cosmic rays within a sample solar rotation period in 2006. By inversely solving the diffusive flux equation, including the drift, we deduce the gradient from the anisotropy that is derived from the observation made by the Global Muon Detector Network (GMDN). The anisotropy obtained by applying a new analysis method to the GMDN data is precise and free from atmospheric temperature effects on the muon count rate recorded by ground-based detectors. We find the derived north-south gradient perpendicular to the ecliptic plane is oriented toward the heliospheric current sheet (HCS; i.e., southward in the toward sector of the interplanetary magnetic field [IMF] and northward in the away sector). The orientation of the gradient component parallel to the ecliptic plane remains similar in both sectors, with an enhancement of its magnitude seen after the Earth crosses the HCS. These temporal features are interpreted in terms of a local maximum of the cosmic ray density at the HCS. This is consistent with the prediction of the drift model for the A<0 epoch. By comparing the observed gradient with the numerical prediction of a simple drift model, we conclude that particle drifts in the large-scale magnetic field play an important role in organizing the density gradient, at least in the present A<0 epoch. We also found that corotating interaction regions did not have such a notable effect. Observations with the GMDN provide us with a new tool for investigating cosmic-ray transport in the IMF.

Compression of magnetic clouds in interplanetary space and increase in their geoeffectiveness

Abstract Using a set of 54 magnetic clouds observed in the period from 1965 to 1997, we found that the magnetic cloud field and velocity strength relationship proposed by Gonzalez et al. (Geophysical Research Letters 25 (7) (1998) 963–966) is generally followed. However, for a few events, the field strength is fairly higher than that predicted by the work of Gonzalez et al. We examined some of these events and found that compression of these magnetic clouds might have occurred, due to the follow-up presence of a higher speed stream, which led to a stronger magnetic field strength at the back region of the cloud. This intensification of the magnetic field strength is closely related to the occurrence of intense geomagnetic storms when the field has a north–south polarity. Such interplanetary compression of magnetic clouds should be considered in space weather forecasting models.

A study of magnetic storms development in two or more steps and its association with the polarity of magnetic clouds

Abstract The study of the response of the magnetosphere, measured by the Dst index, to different interplanetary magnetic field configurations observed in magnetic clouds has led us to conclude that about 20% of intense magnetic storms have the growth of their main phases in three or more steps. We also have found that the magnetic clouds with south-to-north magnetic field rotation tend to lead to moderate or intense magnetic storms with a two-step main phase development due to the closer southward magnetic fields in the sheath and in the cloud. On the other hand, magnetic clouds with a north-to-south magnetic field rotation mostly seem to lead to magnetic storms with one-step main phase development, due to the larger separation between the southward magnetic fields in the sheath and in the cloud. Furthermore, the magnetic clouds with a substantial tilt to the ecliptic plane appear to lead to intense magnetic storms when they have a prolonged southward axial field.

Interplanetary shock parameters during solar activity maximum (2000) and minimum (1995-1996)

Interplanetary shock parameters are analyzed for solar maximum (year 2000) and solar minimum (years 1995-1996) activity. Fast forward shocks are the most usual type of shock observed in the interplanetary medium near Earths orbit, and they are 88% of the identified shocks in 2000 and 60% in 1995-1996. Average plasma and magnetic field parameters for upstream and downstream sides of the shocks were calculated, and the parameter variations through the shock were determined. Applications of the Rankine-Hugoniot equations were made, obtaining shock speeds and Alfvenic Mach number. Static and dynamic pressures variations through the shocks were also calculated. Every parameter have larger variation through the shock in solar maximum than in solar minimum, with exception of the proton density. The intensity of shocks relative to the interplanetary medium, quantified by the Alfvenic Mach Number, is observed to be similar in solar maximum and minimum. It could be explained because, during solar maximum, in despite of the higher shock speeds, the Alfvenic speed of the interplanetary medium is higher than in solar minimum.

Great geomagnetic storms in the rise and maximum of solar cycle 23

Geomagnetic storms are intervals of time when a sufficiently intense and long-lasting interplanetary convection electric field leads, through a substantial injection of energy into the magnetosphere-ionosphere system, to an intensified ring current, strong enough to exceed some key threshold of the quantifying storm time Dst index. We have studied all the 9 great magnetic storms (peak Dst < -200 nT) observed during the rise and maximum of solar cycle 23 (from 1997 to early 2001), in order to identify their solar and interplanetary causes. Apart of one storm occurred during the period without observations from the Solar and Heliospheric Observatory (SOHO), all of them were related to coronal mass ejections observed by the Large Angle and Spectroscopic Coronagraph (LASCO). The sources of interplanetary southward magnetic field, Bs, responsible for the occurrence of the storms were related to the intensified shock/sheath field, interplanetary magnetic clouds field, or the combination of sheath-cloud or sheath-ejecta field. It called our attention the fact that one of the events was related to a slow CME, with CME expansion speed not greater than 550 km/s. The purpose of this paper is to address the main sources of large geomagnetic disturbances using the current satellite capability available. As a general conclusion, we found that shock/sheath compressed fields are the most important interplanetary causes of great magnetic storms during this period.

Outer radiation belt dropout dynamics following the arrival of two interplanetary coronal mass ejections

Magnetopause shadowing and wave-particle interactions are recognized as the two primary mechanisms for losses of electrons from the outer radiation belt. We investigate these mechanisms, sing satellite observations both in interplanetary space and within the magnetosphere and particle drift modeling. Two interplanetary shocks sheaths impinged upon the magnetopause causing a relativistic electron flux dropout. The magnetic cloud (C) and interplanetary structure sunward of the MC had primarily northward magnetic field, perhaps leading to a concomitant lack of substorm activity and a 10 day long quiescent period. The arrival of two shocks caused an unusual electron flux dropout. Test-particle simulations have shown 2 to 5 MeV energy, equatorially mirroring electrons with initial values of L 5.5can be lost to the magnetosheath via magnetopause shadowing alone. For electron losses at lower L-shells, coherent chorus wave-driven pitch angle scattering and ULF wave-driven radial transport have been shownto be viable mechanisms.

Temperature effect correction for the cosmic ray muon data observed at the Brazilian Southern Space Observatory in São Martinho da Serra

The negative atmospheric temperature effect observed in the muon intensity measured by surface-level detectors is related to the atmospheric expansion during summer periods. According the first explanation given, the path of muons from the higher atmospheric level (where they are generated) to the ground becomes longer, and more muons decay, leading to a muon intensity decrease. A significant negative correlation, therefore, is expected between the altitude of the equi-pressure surface and the muon intensity. We compared measurements of the altitude of 100 hPa equi-pressure surface and data from the multidirectional muon detector installed at the Brazilian Southern Space Observatory in Sao Martinho da Serra, RS. Significant correlation coefficient were found (up to 0.95) when using data observed in 2008. For comparison, data from the multidirectional muon detector of Nagoya, located in the opposite hemisphere, is studied and an anti-phase in the cosmic ray variation related with the temperature effect is expected between data from detectors of Nagoya and Sao Martinho da Serra. The temperature influence is higher for the directional channels of Nagoya than for ones of Sao Martinho da Serra.

Coronal mass ejection speeds measured in the solar corona using LASCO C2 and C3 images

Abstract In this work we present height-time diagrams of 2 halo coronal mass ejections, observed on September 28th, 1997 and June 29th, 1999. The CMEs were observed by the Large Angle and Spectroscopic Coronagraph (LASCO), which observes the solar corona from 2 to 32 solar radii. To obtain these diagrams we divide the LASCO images of a given sequence in angular slices, transform them into rectangular slices (their width chosen proportional to the time distance to the next image) and place them side by side. Thus, the speed profile of any pattern moving in the particular latitudinal slice can be derived. With this method we were able to identify even minor speed changes in several angular positions for the chosen events. This technique is particularly appropriate to identify acceleration or deceleration of structures in halo CMEs.