25 research outputs found
Hemispheric sunspot numbers 1874--2020
We create a continuous series of daily and monthly hemispheric sunspot
numbers (HSNs) from 1874 to 2020, which will be continuously expanded in the
future with the HSNs provided by SILSO. Based on the available daily
measurements of hemispheric sunspot areas from 1874 to 2016 from Greenwich
Royal Observatory and NOAA, we derive the relative fractions of the northern
and southern activity. These fractions are applied to the international sunspot
number (ISN) to derive the HSNs. This method and obtained data are validated
against published HSNs for the period 1945--2020. We provide a continuous data
series and catalogue of daily, monthly mean, and 13-month smoothed monthly mean
HSNs for the time range 1874--2020 that are consistent with the newly
calibrated ISN. Validation of the reconstructed HSNs against the direct data
available since 1945 reveals a high level of consistency, with a correlation of
r=0.94 (0.97) for the daily (monthly) data. The cumulative hemispheric
asymmetries for cycles 12-24 give a mean value of 16%, with no obvious pattern
in north-south predominance over the cycle evolution. The strongest asymmetry
occurs for cycle no. 19, in which the northern hemisphere shows a cumulated
predominance of 42%. The phase shift between the peaks of solar activity in the
two hemispheres may be up to 28 months, with a mean absolute value of 16.4
months. The phase shifts reveal an overall asymmetry of the northern hemisphere
reaching its cycle maximum earlier (in 10 out of 13 cases). Relating the ISN
and HSN peak growth rates during the cycle rise phase with the cycle amplitude
reveals higher correlations when considering the two hemispheres individually,
with r = 0.9. Our findings demonstrate that empirical solar cycle prediction
methods can be improved by investigating the solar cycle dynamics in terms of
the hemispheric sunspot numbers.Comment: Accepted by Astron. Astrophys. 12 page
Genesis and impulsive evolution of the 2017 September 10 coronal mass ejection
The X8.2 event of 10 September 2017 provides unique observations to study the
genesis, magnetic morphology and impulsive dynamics of a very fast CME.
Combining GOES-16/SUVI and SDO/AIA EUV imagery, we identify a hot ( MK) bright rim around a quickly expanding cavity, embedded inside a much
larger CME shell ( MK). The CME shell develops from a dense set
of large AR loops (0.5 ), and seamlessly evolves into the CME
front observed in LASCO C2. The strong lateral overexpansion of the CME shell
acts as a piston initiating the fast EUV wave. The hot cavity rim is
demonstrated to be a manifestation of the dominantly poloidal flux and
frozen-in plasma added to the rising flux rope by magnetic reconnection in the
current sheet beneath. The same structure is later observed as the core of the
white light CME, challenging the traditional interpretation of the CME
three-part morphology. The large amount of added magnetic flux suggested by
these observations explains the extreme accelerations of the radial and lateral
expansion of the CME shell and cavity, all reaching values of km
s. The acceleration peaks occur simultaneously with the first RHESSI
keV hard X-ray burst of the associated flare, further underlining the
importance of the reconnection process for the impulsive CME evolution.
Finally, the much higher radial propagation speed of the flux rope in relation
to the CME shell causes a distinct deformation of the white light CME front and
shock.Comment: Accepted for publication in the Astrophysical Journa
Advancing solar magnetic field extrapolations through multi-height magnetic field measurements
Non-linear force-free extrapolations are a common approach to estimate the 3D
topology of coronal magnetic fields based on photospheric vector magnetograms.
The force-free assumption is a valid approximation at coronal heights, but for
the dense plasma conditions in the lower atmosphere, this assumption is not
satisfied. In this study, we utilize multi-height magnetic field measurements
in combination with physics-informed neural networks to advance solar magnetic
field extrapolations. We include a flexible height-mapping, which allows us to
account for the different formation heights of the observed magnetic field
measurements. The comparison to analytical and simulated magnetic fields
demonstrates that including chromospheric magnetic field measurements leads to
a significant improvement of our magnetic field extrapolations. We also apply
our method to chromospheric line-of-sight magnetograms, from the Vector
Spectromagnetograph (VSM) on the Synoptic Optical Long-term Investigations of
the Sun (SOLIS) observatory, in combination with photospheric vector
magnetograms, from the Helioseismic Magnetic Imager (HMI) onboard the Solar
Dynamic Observatory (SDO). The comparison to observations in extreme
ultraviolet wavelengths shows that the additional chromospheric information
leads to a better agreement with the observed coronal structures. In addition,
our method intrinsically provides an estimate of the corrugation of the
observed magnetograms. With this new approach, we make efficient use of
multi-height magnetic field measurements and advance the realism of coronal
magnetic field simulations