Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userA homogeneous aa index: 2. hemispheric asymmetries and the equinoctial variation
387
0
0.0
(
0
)
Added by
Mike Lockwood Prof.
Publication date
2018
fields
Physics
and research's language is
English
Ask ChatGPT about the research
No Arabic abstract
Paper 1 [Lockwood et al., 2018] generated annual means of a new version of the $aa$ geomagnetic activity index which includes corrections for secular drift in the geographic coordinates of the auroral oval, thereby resolving the difference between the centennial-scale change in the northern and southern hemisphere indices, $aa_N$ and $aa_S$. However, other hemispheric asymmetries in the $aa$ index remain: in particular, the distributions of 3-hourly $aa_N$ and $aa_S$ values are different and the correlation between them is not high on this timescale ($r = 0.66$). In the present paper, a location-dependant station sensitivity model is developed using the $am$ index (derived from a much more extensive network of stations in both hemispheres) and used to reduce the difference between the hemispheric $aa$ indices and improve their correlation (to $r = 0.79$) by generating corrected 3-hourly hemispheric indices, $aa_{HN}$ and $aa_{HS}$, which also include the secular drift corrections detailed in Paper 1. These are combined into a new, homogeneous $aa$ index, $aa_H$. It is shown that $aa_H$, unlike $aa$, reveals the equinoctial-like time-of-day/time-of-year pattern that is found for the $am$ index.
rate research
Read More
Originally complied for 1868-1967 and subsequently continued so that it now covers 150 years, the $aa$ index has become a vital resource for studying space climate change. However, there have been debates about the inter-calibration of data from the different stations. In addition, the effects of secular change in the geomagnetic field have not previously been allowed for. As a result, the components of the classical $aa$ index for the southern and northern hemispheres ($aa_S$ and $aa_N$) have drifted apart. We here separately correct both $aa_S$ and $aa_N$ for both these effects using the same method as used to generate the classic $aa$ values but allowing ${delta}$, the minimum angular separation of each station from a nominal auroral oval, to vary as calculated using the IGRF-12 and gufm1 models of the intrinsic geomagnetic field. Our approach is to correct the quantized aK-values for each station, originally scaled on the assumption that ${delta}$ values are constant, with time-dependent scale factors that allow for the drift in ${delta}$. This requires revisiting the intercalibration of successive stations used in making the $aa_S$ and $aa_N$ composites. These intercalibrations are defined using independent data and daily averages from 11 years before and after each station change and it is shown that they depend on the time of year. This procedure produces new homogenized hemispheric aa indices, $aa_{HS}$ and $aa_{HN}$, which show centennial-scale changes that are in very close agreement. Calibration problems with the classic $aa$ index are shown to have arisen from drifts in ${delta}$ combined with simpler corrections which gave an incorrect temporal variation and underestimate the rise in $aa$ during the 20th century by about 15%.
We study the relationship between solar wind helium to hydrogen abundance ratio ($A_mathrm{He}$), solar wind speed ($v_mathrm{SW}$), and sunspot number (SSN) over solar cycles 23 and 24. This is the first full 22-year Hale cycle measured with the Wind spacecraft covering a full cycle of the solar dynamo with two polarity reversals. While previous studies have established a strong correlation between $A_mathrm{He}$ and SSN, we show that the phase delay between $A_mathrm{He}$ and SSN is a monotonic increasing function of $v_mathrm{SW}$. Correcting for this lag, $A_mathrm{He}$ returns to the same value at a given SSN over all rising and falling phases and across solar wind speeds. We infer that this speed-dependent lag is a consequence of the mechanism that depletes slow wind $A_mathrm{He}$ from its fast wind value during solar wind formation.
We report ground truth, 28-3500 keV in-situ ion and 5.2-55 keV remotely sensed ENA measurements from Voyager 2/Low Energy Charged Particle (LECP) detector and Cassini/Ion and Neutral Camera (INCA), respectively, that assess the components of the ion pressure in the heliosheath. In this process, we predict an interstellar neutral hydrogen density of ~0.12 cm-3 and an interstellar magnetic field strength of ~0.5 nT upstream of the heliopause in the direction of V2, i.e. consistent with the measured magnetic field and neutral density measurements at Voyager 1 from August 2012, when the spacecraft entered interstellar space, to date. Further, this analysis results in an estimated heliopause crossing by V2 of ~119 AU, as observed, suggesting that the parameters deduced from the pressure analysis are valid. The shape of the >5.2 keV ion energy spectra play a critical role towards determining the pressure balance and acceleration mechanisms inside the heliosheath.
Among many other measurable quantities the summer of 2009 saw a considerable low in the radiative output of the Sun that was temporally coincident with the largest cosmic ray flux ever measured at 1AU. A hemispheric asymmetry in magnetic activity is clearly observed and its evolution monitored and the resulting (prolonged) magnetic imbalance must have had a considerable impact on the structure and energetics of the heliosphere. While we cannot uniquely tie the variance and scale of the surface magnetism to the dwindling radiative and particulate output of the star, or the increased cosmic ray flux through the 2009 minimum, the timing of the decline and rapid recovery in early 2010 would appear to inextricably link them. These observations support a picture where the Suns hemispheres are significantly out of phase with each other. Studying historical sunspot records with this picture in mind shows that the northern hemisphere has been leading since the middle of the last century and that the hemispheric dominance has changed twice in the past 130 years. The observations presented give clear cause for concern, especially with respect to our present understanding of the processes that produce the surface magnetism in the (hidden) solar interior - hemispheric asymmetry is the normal state - the strong symmetry shown in 1996 was abnormal. Further, these observations show that the mechanism(s) which create and transport the magnetic flux magnetic flux are slowly changing with time and, it appears, with only loose coupling across the equator such that those asymmetries can persist for a considerable time. As the current asymmetry persists and the basal energetics of the system continue to dwindle we anticipate new radiative and particulate lows coupled with increased cosmic ray fluxes heading into the next solar minimum.
We present observations of the same magnetic cloud made near Earth by the Advance Composition Explorer (ACE), Wind, and the Acceleration, Reconnection, Turbulence and Electrodynamics of the Moons Interaction with the Sun (ARTEMIS) mission comprising the Time History of Events and Macroscale Interactions during Substorms (THEMIS) B and THEMIS C spacecraft, and later by Juno at a distance of 1.2 AU. The spacecraft were close to radial alignment throughout the event, with a longitudinal separation of $3.6^{circ}$ between Juno and the spacecraft near Earth. The magnetic cloud likely originated from a filament eruption on 22 October 2011 at 00:05 UT, and caused a strong geomagnetic storm at Earth commencing on 24 October. Observations of the magnetic cloud at each spacecraft have been analysed using Minimum Variance Analysis and two flux rope fitting models, Lundquist and Gold-Hoyle, to give the orientation of the flux rope axis. We explore the effect different trailing edge boundaries have on the results of each analysis method, and find a clear difference between the orientations of the flux rope axis at the near-Earth spacecraft and Juno, independent of the analysis method. The axial magnetic field strength and the radial width of the flux rope are calculated using both observations and fitting parameters and their relationship with heliocentric distance is investigated. Differences in results between the near-Earth spacecraft and Juno are attributed not only to the radial separation, but to the small longitudinal separation which resulted in a surprisingly large difference in the in situ observations between the spacecraft. This case study demonstrates the utility of Juno cruise data as a new opportunity to study magnetic clouds beyond 1 AU, and the need for caution in future radial alignment studies.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
