Two populations of twisted magnetic field tubes, or flux ropes (hereafter, FRs), are detected by in situ measurements in the solar wind. While small FRs are crossed by the observing spacecraft within few hours, with a radius typically less than 0.1AU
, larger FRs, or magnetic clouds (hereafter, MCs), have durations of about half a day. The main aim of this study is to compare the properties of both populations of FRs observed by the Wind spacecraft at 1 AU. To do so, we use standard correlation techniques for the FR parameters, as well as histograms and more refined statistical methods. Although several properties seem at first different for small FRs and MCs, we show that they are actually governed by the same propagation physics. For example, we observe no in situ signatures of expansion for small FRs, contrary to MCs. We demonstrate that this result is in fact expected: small FRs expand similarly to MCs, as a consequence of a total pressure balance with the surrounding medium, but the expansion signature is well hidden by velocity fluctuations. Next, we find that the FR radius, velocity and magnetic field strength are all positively correlated, with correlation factors than can reach a value >0.5. This result indicates a remnant trace of the FR ejection process from the corona. We also find a larger FR radius at the apex than at the legs (up to three times larger at the apex), for FR observed at 1 AU. Finally, assuming that the detected FRs have a large-scale configuration in the heliosphere, we derived the mean axis shape from the probability distribution of the axis orientation. We therefore interpret the small FR and MC properties in a common framework of FRs interacting with the solar wind, and we disentangle the physics present behind their common and different features.
The impact of the solar activity on the heliosphere has a strong influence on the modulation of the flux of low energy galactic cosmic rays arriving at Earth. Different instruments, such as neutron monitors or muon detectors, have been recording the
variability of the cosmic ray flux at ground level for several decades. Although the Pierre Auger Observatory was designed to observe cosmic rays at the highest energies, it also records the count rates of low energy secondary particles (the scaler mode) for the self-calibration of its surface detector array. From observations using the scaler mode at the Pierre Auger Observatory, modulation of galactic cosmic rays due to solar transient activity has been observed (e.g., Forbush decreases). Due to the high total count rate coming from the combined area of its detectors, the Pierre Auger Observatory (its detectors have a total area greater than $16,000$,m$^2$) detects a flux of secondary particles of the order of $sim 10^8$,counts per minute. Time variations of the cosmic ray flux related to the activity of the heliosphere can be determined with high accuracy. In this paper we briefly describe the scaler mode and analyze a Forbush decrease together with the interplanetary coronal mass ejection that originated it. The Auger scaler data are now publicly available.
We analyze the evolution of the interplanetary magnetic field spatial structure by examining the inner heliospheric autocorrelation function, using Helios 1 and Helios 2 in situ observations. We focus on the evolution of the integral length scale (la
mbda) anisotropy associated with the turbulent magnetic fluctuations, with respect to the aging of fluid parcels traveling away from the Sun, and according to whether the measured lambda is principally parallel (lambda_parallel) or perpendicular (lambda_perp) to the direction of a suitably defined local ensemble average magnetic field B0. We analyze a set of 1065 24-hour long intervals (covering full missions). For each interval, we compute the magnetic autocorrelation function, using classical single-spacecraft techniques, and estimate lambda with help of two different proxies for both Helios datasets. We find that close to the Sun, lambda_parallel < lambda_perp. This supports a slab-like spectral model, where the population of fluctuations having wavevector k parallel to B0 is much larger than the one with k-vector perpendicular. A population favoring perpendicular k-vectors would be considered quasi-two dimensional (2D). Moving towards 1 AU, we find a progressive isotropization of lambda and a trend to reach an inverted abundance, consistent with the well-known result at 1 AU that lambda_parallel > lambda_perp, usually interpreted as a dominant quasi-2D picture over the slab picture. Thus, our results are consistent with driving modes having wavevectors parallel to B0 near Sun, and a progressive dynamical spectral transfer of energy to modes with perpendicular wavevectors as the solar wind parcels age while moving from the Sun to 1 AU.
