Ultra-high-resolution observations of persistent null-point reconnection in the solar corona.

Magnetic reconnection is a key mechanism involved in solar eruptions and is also a prime possibility to heat the low corona to millions of degrees. Here, we present ultra-high-resolution extreme ultraviolet observations of persistent null-point reconnection in the corona at a scale of about 390 km over one hour observations of the Extreme-Ultraviolet Imager on board Solar Orbiter spacecraft. The observations show formation of a null-point configuration above a minor positive polarity embedded within a region of dominant negative polarity near a sunspot. The gentle phase of the persistent null-point reconnection is evidenced by sustained point-like high-temperature plasma (about 10 MK) near the null-point and constant outflow blobs not only along the outer spine but also along the fan surface. The blobs appear at a higher frequency than previously observed with an average velocity of about 80 km s-1 and life-times of about 40 s. The null-point reconnection also occurs explosively but only for 4 minutes, its coupling with a mini-filament eruption generates a spiral jet. These results suggest that magnetic reconnection, at previously unresolved scales, proceeds continually in a gentle and/or explosive way to persistently transfer mass and energy to the overlying corona.

Unusual activity of the Sun during recent decades compared to the previous 11,000 years.

Direct observations of sunspot numbers are available for the past four centuries, but longer time series are required, for example, for the identification of a possible solar influence on climate and for testing models of the solar dynamo. Here we report a reconstruction of the sunspot number covering the past 11,400 years, based on dendrochronologically dated radiocarbon concentrations. We combine physics-based models for each of the processes connecting the radiocarbon concentration with sunspot number. According to our reconstruction, the level of solar activity during the past 70 years is exceptional, and the previous period of equally high activity occurred more than 8,000 years ago. We find that during the past 11,400 years the Sun spent only of the order of 10% of the time at a similarly high level of magnetic activity and almost all of the earlier high-activity periods were shorter than the present episode. Although the rarity of the current episode of high average sunspot numbers may indicate that the Sun has contributed to the unusual climate change during the twentieth century, we point out that solar variability is unlikely to have been the dominant cause of the strong warming during the past three decades.

Three-dimensional magnetic field topology in a region of solar coronal heating.

Flares and X-ray jets on the Sun arise in active regions where magnetic flux emerges from the solar interior amd interacts with the ambient magnetic field. The interactions are believed to occur in electric current sheets separating regions of opposite magnetic polarity. The current sheets located in the corona or upper chromosphere have long been thought to act as an important source of coronal heating, requiring their location in the corona or upper chromosphere. The dynamics and energetics of these sheets are governed by a complex magnetic field structure that, until now, has been difficult to measure. Here we report the determination of the full magnetic vector in an interaction region near the base of the solar corona. The observations reveal two magnetic features that characterize young active regions on the Sun: a set of rising magnetic loops and a tangential discontinuity of the magnetic field direction, the latter being the observational signature of an electric current sheet. This provides strong support for coronal heating models based on the dissipation of magnetic energy at current sheets.

New light on the heart of darkness of the solar chromosphere.

Solar carbon monoxide spectra indicate the existence of a cool (less than 4000 kelvin) component to the solar chromosphere coexisting with the hot, bright gas at 6000 to 7000 kelvin. However, both the existence and the location of the cool component have been controversial. New high-resolution spectra show that carbon monoxide goes into emission just beyond the limb, allowing it to be probed without photospheric contamination. The cool component has temperatures as low as 3000 to 3500 kelvin and appears to cover 50 to 85 percent of the quiet solar surface. There is a steep temperature rise to normal chromospheric temperatures at a height of 900 to 1100 kilometers. Large horizontal velocities are seen, suggesting that the cool component is maintained by the supersonic adiabatic expansion of upwelling gas in overshooting granules.

The Laschamp geomagnetic excursion featured in nitrate record from EPICA-Dome C ice core.

Here we present the first direct comparison of cosmogenic (10)Be and chemical species in the period of 38-45.5 kyr BP spanning the Laschamp geomagnetic excursion from the EPICA-Dome C ice core. A principal component analysis (PCA) allowed to group different components as a function of the main sources, transport and deposition processes affecting the atmospheric aerosol at Dome C. Moreover, a wavelet analysis highlighted the high coherence and in-phase relationship between (10)Be and nitrate at this time. The evident preferential association of (10)Be with nitrate rather than with other chemical species was ascribed to the presence of a distinct source, here labelled as "cosmogenic". Both the PCA and wavelet analyses ruled out a significant role of calcium in driving the (10)Be and nitrate relationship, which is particularly relevant for a plateau site such as Dome C, especially in the glacial period during which the Laschamp excursion took place. The evidence that the nitrate record from the EDC ice core is able to capture the Laschamp event hints toward the possibility of using this marker for studying galactic cosmic ray flux variations and thus also major geomagnetic field excursions at pluri-centennial-millennial time scales, thus opening up new perspectives in paleoclimatic studies.

Evolution of the Sun's large-scale magnetic field since the Maunder minimum.

The most striking feature of the Sun's magnetic field is its cyclic behaviour. The number of sunspots, which are dark regions of strong magnetic field on the Sun's surface, varies with a period of about 11 years. Superposed on this cycle are secular changes that occur on timescales of centuries and events like the Maunder minimum in the second half of the seventeenth century, when there were very few sunspots. A part of the Sun's magnetic field reaches out from the surface into interplanetary space, and it was recently discovered that the average strength of this interplanetary field has doubled in the past 100 years. There has hitherto been no clear explanation for this doubling. Here we present a model describing the long-term evolution of the Sun's large-scale magnetic field, which reproduces the doubling of the interplanetary field. The model indicates that there is a direct connection between the length of the sunspot cycle and the secular variations.

Intercalibration of SUMER and CDS on SOHO. II. SUMER Detectors A and B and CDS NIS.

Results of an intercalibration between the extreme-ultraviolet spectrometers Coronal Diagnostic Spectrometer (CDS) and Solar Ultraviolet Measurements of Emitted Radiation (SUMER) on board the Solar and Heliospheric Observatory (SOHO) are reported. The results of the joint observing program Intercal_01 are described, and intercalibration results up to July 2000 of both SUMER detectors A and B and of the CDS Normal Incidence Spectrometer (NIS) are presented. The instruments simultaneously observed radiance of emission lines at the center of the Sun, and three lines have been chosen for intercomparison: He i 584 A, Mg x 609 A, and Mg x 624 A. Initially the same area was observed by both instruments, but, after restrictions were imposed by the scanning mechanism of SUMER in November 1996, the instruments viewed areas of different sizes. Nevertheless, the temporal correlation between the two instruments remained good through June 1998, when contact with the SOHO spacecraft was lost. Until then the CDS instrument measured (33 ? 5)% and (38 ? 7)% (?1varsigma) higher intensity than SUMER in the He i 584-A line on average for detectors A and B, respectively. Data from SUMER detector B agreed well for Mg x 609 A and Mg x 624 A with the CDS intensities, showing offsets of (2 ? 10)% and (9 ? 15)%, much less than the data of detector A with offsets of (7 ? 8)% and (16 ? 7)% for the two lines, respectively, relative to CDS. Finally, the intercalibration measurements after the loss and recovery of the SOHO spacecraft are analyzed. The data for observations from November 1998 to July 2000 are compared, and it is shown that, although the responses of the instruments have changed, the CDS and the SUMER still perform well, and their temporal correlation is good.

Intercalibration of SUMER and CDS on SOHO. I. SUMER detector A and CDS NIS.

The results of an intercalibration between the extreme ultraviolet spectrometers Coronal Diagnostic Spectrometer (CDS) and Solar Ultraviolet Measurements of Emitted Radiation (SUMER) onboard the Solar and Heliospheric Observatory (SOHO) are presented. During the joint observing program Intercal_01, CDS and SUMER were pointed at the same locations in quiet Sun areas and observed in the same wavelength bands located around the spectral lines He i 584 A, Mg x 609 A, and Mg x 624 A. The data sets analyzed here consist of raster images recorded by the CDS normal-incidence spectrometer and SUMER detector A and span the time from March 1996 to August 1996. Effects of the different spatial and spectral resolutions of both instruments have been investigated and taken into account in the analysis. We find that CDS measures generally a 30% higher intensity than SUMER in the He i 584-A line, while it measures 9% and 17% higher intensities in Mg x 609 A and Mg x 624 A, respectively. Both instruments show very good temporal correlation and stability, indicating that solar variations dominate over changes in instrumental sensitivity. Our analysis also provides in-flight estimates of the CDS spatial point-spread functions.