Hardi Peter

Planck 2013 results

In this volume, we proudly present a special feature on science results from the data that ESA Planck mission gathered over its first 15 months and which ESA and the Planck Collaboration released in March 2013. This collection of 31 articles presents the initial scientific results extracted from this first Planck dataset, which measures the cosmic microwave background (CMB) with the highest accuracy to date. It provides major new advances in different domains of cosmology and astrophysics. We thank Jan Tauber and the Planck Science Team for coordinating this special feature.

Prevalence of small-scale jets from the networks of the solar transition region and chromosphere

As the interface between the Sun’s photosphere and corona, the chromosphere and transition region play a key role in the formation and acceleration of the solar wind. Observations from the Interface Region Imaging Spectrograph reveal the prevalence of intermittent small-scale jets with speeds of 80 to 250 kilometers per second from the narrow bright network lanes of this interface region. These jets have lifetimes of 20 to 80 seconds and widths of ≤300 kilometers. They originate from small-scale bright regions, often preceded by footpoint brightenings and accompanied by transverse waves with amplitudes of ~20 kilometers per second. Many jets reach temperatures of at least ~105 kelvin and constitute an important element of the transition region structures. They are likely an intermittent but persistent source of mass and energy for the solar wind.

Hot explosions in the cool atmosphere of the Sun

The solar atmosphere was traditionally represented with a simple one-dimensional model. Over the past few decades, this paradigm shifted for the chromosphere and corona that constitute the outer atmosphere, which is now considered a dynamic structured envelope. Recent observations by the Interface Region Imaging Spectrograph (IRIS) reveal that it is difficult to determine what is up and down, even in the cool 6000-kelvin photosphere just above the solar surface: This region hosts pockets of hot plasma transiently heated to almost 100,000 kelvin. The energy to heat and accelerate the plasma requires a considerable fraction of the energy from flares, the largest solar disruptions. These IRIS observations not only confirm that the photosphere is more complex than conventionally thought, but also provide insight into the energy conversion in the process of magnetic reconnection.

Intermittent heating in the solar corona employing a 3D MHD model

Aims. We investigate the spatial and temporal evolution of the heating of the corona of a cool star such as our Sun in a threedimensional magneto-hydrodynamic (3D MHD) model. Methods. We solve the 3D MHD problem numerically in a box representing part of the (solar) corona. The energy balance includes Spitzer heat conduction along the magnetic field and optically thin radiative losses. The self-consistent heating mechanism is based on the braiding of magnetic field lines rooted in the convective photosphere. Magnetic stress induced by photospheric motions leads to currents in the atmosphere that heat the corona through Ohmic dissipation. Results. While the horizontally averaged quantities, such as heating rate, temperature, or density, are relatively constant in time, the simulated corona is highly variable and dynamic, on average reaching the temperatures and densities found in observations. The strongest heating per particle is found in the transition region from the chromosphere to the corona. The heating is concentrated in current sheets roughly aligned with the magnetic field and is transient in time and space. This supports the idea that numerous small heating events heat the corona, often referred to as nanoflares.

Structure of solar coronal loops: from miniature to large-scale

Aims. We will use new data from the High-resolution Coronal Imager (Hi-C) with unprecedented spatial resolution of the solar corona to investigate the structure of coronal loops down to 0.2′′. Methods. During a rocket flight Hi-C provided images of the solar corona in a wavelength band around 193 Å that is dominated by emission from Fe XII showing plasma at temperatures around 1.5 MK. We analyze part of the Hi-C field-of-view to study the smallest coronal loops observed so far and search for the a possible sub-structuring of larger loops. Results. We find tiny 1.5 MK loop-like structures that we interpret as miniature coronal loops. These have length of the coronal segment above the chromosphere of only about 1 Mm and a thickness of less than 200 km. They could be interpreted as the coronal signature of small flux tubes breaking through the photosphere with a footpoint distance corresponding to the diameter of a cell of granulation. We find loops that are longer than 50 Mm to have a diameter of about 2′′ or 1.5 Mm, consistent with previous observations. However, Hi-C really resolves these loops with some 20 pixels across the loop. Even at this greatly improved spatial resolution the large loops seem to have no visible sub-structure. Instead they show a smooth variation in cross-section. Conclusions. The fact that the large coronal loops do not show a sub-structure at the spatial scale of 0.1′′ per pixel implies that either the densities and temperatures are smoothly varying across these loops or poses an upper limit on the diameter of strands the loops might be composed of. We estimate that strands that compose the 2′′ thick loop would have to be thinner than 15 km. The miniature loops we find for the first time pose a challenge to be properly understood in terms of modeling.

Asymmetries of solar coronal extreme ultraviolet emission lines

Context. The profiles of emission lines formed in the corona contain information on the dynamics and the heating of the hot plasma. Only recently has data with sufficiently high spectral resolution become available for investigating the details of the profiles of emission lines formed well above 10 6 K. These show enhanced emission in the line wings, which has not been understood yet. Aims. We study the underlying processes leading to asymmetric line profiles, in particular the responsible plasma flows and line broadening mechanisms in a highly filamentary and dynamic atmosphere. Methods. Line profiles of Fe xv formed at 2.5 MK acquired by the Extreme ultraviolet Imaging Spectrometer (EIS) onboard the Hinode solar space observatory are studied using multi Gaussian fits, with emphasis on the resulting line widths and Doppler shifts. Results. In the major part of the active region, the spectra are best fit by a narrow line core and a broad minor component. The latter contributes some 10% to 20% to the total emission, is about a factor of 2 broader than the core, and shows strong blueshifts of up to 50 km s −1 , especially in the footpoint regions of the loops. On average, the line width increases from the footpoints to the loop top for both components. A component with high upflow speeds can be found also in small restricted areas. Conclusions. The coronal structures consist of at least two classes that are not resolved spatially but only spectroscopically and that are associated with the line core and the minor component. Because of their huge line width and strong upflows, it is proposed that the major part of the heating and the mass supply to the corona is actually located in source regions of the minor component. It might be that these are identical to type II spicules. The siphon flows and draining loops seen in the line core component are consistent with structures found in a three-dimensional magneto-hydrodynamic (3D MHD) coronal model. Despite the quite different appearance of the large active region corona and small network elements seen in transition region lines, both show similar line profile characteristics. This indicates that the same processes govern the heating and dynamics of the transition region and the corona.

Persistent Doppler Shift Oscillations Observed with Hinode/EIS in the Solar Corona: Spectroscopic Signatures of Alfvénic Waves and Recurring Upflows

Using data obtained by the EUV Imaging Spectrometer on board Hinode, we have performed a survey of obvious and persistent (without significant damping) Doppler shift oscillations in the corona. We have found mainly two types of oscillations from February to April in 2007. One type is found at loop footpoint regions, with a dominant period around 10 minutes. They are characterized by coherent behavior of all line parameters (line intensity, Doppler shift, line width, and profile asymmetry), and apparent blueshift and blueward asymmetry throughout almost the entire duration. Such oscillations are likely to be signatures of quasi-periodic upflows (small-scale jets, or coronal counterpart of type-II spicules), which may play an important role in the supply of mass and energy to the hot corona. The other type of oscillation is usually associated with the upper part of loops. They are most clearly seen in the Doppler shift of coronal lines with formation temperatures between one and two million degrees. The global wavelets of these oscillations usually peak sharply around a period in the range of three to six minutes. No obvious profile asymmetry is found and the variation of the line width is typically very small. The intensity variation is often less than 2%. These oscillations are more likely to be signatures of kink/Alfv´ en waves rather than flows. In a few cases, there seems to be a π/ 2 phase shift between the intensity and Doppler shift oscillations, which may suggest the presence of slow-mode standing waves according to wave theories. However, we demonstrate that such a phase

Numerical Simulations of Chromospheric Anemone Jets Associated with Moving Magnetic Features

Observations with the space-based solar observatory Hinode show that small-scale magnetic structures in the photosphere are found to be associated with a particular class of jets of plasma in the chromosphere called anemone jets. The goal of our study is to conduct a numerical experiment of such chromospheric anemone jets related to the moving magnetic features (MMFs). We construct a 2.5 dimensional numerical MHD model to describe the process of magnetic reconnection between the MMFs and the pre-existing ambient magnetic field, which is driven by the horizontal motion of the magnetic structure in the photosphere. We include thermal conduction parallel to the magnetic field and optically thin radiative losses in the corona to account for a self-consistent description of the evaporation process during the heating of the plasma due to the reconnection process. The motion of the MMFs leads to the expected jet and our numerical results can reproduce many observed characteristics of chromospheric anemone jets, topologically and quantitatively. As a result of the tearing instability, plasmoids are generated in the reconnection process that are consistent with the observed bright moving blobs in the anemone jets. An increase in the thermal pressure at the base of the jet is also driven by the reconnection, which induces a train of slow-mode shocks propagating upward. These shocks are a secondary effect, and only modulate the outflow of the anemone jet. The jet itself is driven by the energy input due to the reconnection of the MMFs and the ambient magnetic field.

Constant cross section of loops in the solar corona

Context. The corona of the Sun is dominated by emission from loop-like structures. When observed in X-ray or extreme ultraviolet emission, these million K hot coronal loops show a more or less constant cross section. Aims. In this study we show how the interplay of heating, radiative cooling, and heat conduction in an expanding magnetic structure can explain the observed constant cross section. Methods. We employ a three-dimensional magnetohydrodynamics (3D MHD) model of the corona. The heating of the coronal plasma is the result of braiding of the magnetic field lines through footpoint motions and subsequent dissipation of the induced currents. From the model we synthesize the coronal emission, which is directly comparable to observations from, e.g., the Atmospheric Imaging Assembly on the Solar Dynamics Observatory (AIA/SDO). Results. We find that the synthesized observation of a coronal loop seen in the 3D data cube does match actually observed loops in count rate and that the cross section is roughly constant, as observed. The magnetic field in the loop is expanding and the plasma density is concentrated in this expanding loop; however, the temperature is not constant perpendicular to the plasma loop. The higher temperature in the upper outer parts of the loop is so high that this part of the loop is outside the contribution function of the respective emission line(s). In effect, the upper part of the plasma loop is not bright and thus the loop actually seen in coronal emission appears to have a constant width. Conclusions. From this we can conclude that the underlying field-line-braiding heating mechanism provides the proper spatial and temporal distribution of the energy input into the corona – at least on the observable scales.

Observationally driven 3D magnetohydrodynamics model of the solar corona above an active region

Institut fu¨r Astrophysik, Universita¨t Go¨ttingen, Friedrich-Hund-Platz 1, 37077 Go¨ttingen, GermanyReceived 29 January 2013 / Accepted 9 May 2013ABSTRACTAims. The goal is to employ a 3D magnetohydrodynamics (MHD) model including spectral synthesis to model the corona in anobserved solar active region. This will allow us to judge the merits of the coronal heating mechanism built into the 3D model.Methods.Photospheric observations of the magnetic field and horizon tal velocities in an active region are used to drive our coronalsimulation from the bottom. The currents induced by this heat the corona through Ohmic dissipation. Heat conduction redistributes theenergy that is lost in the end through optically thin radiation. Based on the MHD model, we synthesized profiles of coronal emissionlines which can be directly compared to actual coronal observations of the very same active region.Results.In the synthesized model data we find hot coronal loops which h ost siphon flows or which expand and lose mass throughdraining. These synthesized loops are at the same location as and show similar dynamics in terms of Doppler shifts to the observedstructures. This match is shown through a comparison with Hinode data as well as with 3D stereoscopic reconstructions of data fromSTEREO.Conclusions.The considerable match to the actual observations shows that the field-line braiding mechanism leading to the energyinput in our corona provides the proper distribution of heat input in space and time. From this we conclude that in an active region thefield-line braiding is the dominant heating process, at least at the spatial scales available to current observations.Key words. Sun: corona — Magnetohydrodynamics (MHD) — Methods: numeri cal — Sun: UV radiation