© 2008 Spektrum der Wissenschaft/Emde-Grafik

A LOFAR station consists of two arrays of antennae for low (30 - 80 MHz) and high (120 - 240 MHz) frequencies, respectively. The antennae are simple dipole antennae. The gap between 80 and 120 MHz avoids the FM radio range that inhibits observations with its strong background signals.

In "classical" radio interferometers an array of radio dish antennae is oriented towards the observed source in the sky. The antenna signals are correlated with well-defined delays, and an image is synthesized. With this setup, only a single source can be observed at a certain frequency at the same time. LOFAR pursues a different approach that offers an unprecedented flexibility and versatility. LOFAR uses arrays of simple dipole antennae that essentially cover the whole sky. At each LOFAR station, the antenna signals are digitized, pre-processed and sent to the Central Processing System (CPS) at Groningen, where signal correlation and further data processing is done. This design provides LOFAR with high flexibility and enables the simultaneous observations of up to 8 different sources in the sky. The long baselines of LOFAR provide an angular resolution in the arc-second range, and the large collecting area of all LOFAR stations combined leads to a sensitivity down to a mJy. Thus, LOFAR considerably exceeds the capabilities of current radio telescopes both in resolution and sensitivity.

At low frequencies, only a few instruments with imaging capability observe the Sun. E.g., the radioheliograph at Nancay, France, is operated on selected frequencies ranging from 150 - 432 MHz and yields an image resolution of up to 1'. LOFAR will dramatically improve the situation with routine observations, and will expand the frequency range down to 30 MHz. With simultaneous observations at several frequencies, it will be possible to probe different heights in the corona, and to reveal structures in coronal radio sources that are yet unexplored. LOFAR's flexibility enables different observation modes that are tailored to different scientific objectives:

• Response to solar radio bursts
  Since the control of LOFAR is basically realized in software, it is possible to react quickly on changes in the solar activity. If a continuously operated solar radio spectrometer, e.g. the Observatory for Solar Radioastronomy, detects a radio burst ("burst bell"), follow-up observations with LOFAR can be triggered, e.g. a rapid sequence of images in different frequencies.
• Routine monitoring of the solar activity
  Continuous monitoring of the Sun allows for studies of the long-term evolution of solar active regions and provides information on the precursors of solar radio bursts, flares, and CMEs. Continuous imaging would comprise an sequence of 1 image per minute. Such observations can be ideally combined with optical images, like the H alpha images of the Kanzelhöhe Solar Observatory.
• Space Weather studies
  The phenomena of the active Sun, like flares and CMEs, can have severe impact on the terrestrial environment. This relationship is usually referred to as Space Weather. LOFAR's frequency range covers radio source locations in the upper corona, this is the region where CMEs start towards interplanetary space. With LOFAR images of a nascent CME it is possible to estimate its potential impact on Earth.
• Observation campaigns
  Simultaneous observations at different wavelength ranges are useful for a better understanding of the physical processes in flares and CMEs. Highly energetic electrons in the solar atmosphere do not only emit radio waves, but also X-rays, while the heating of the corona in a flare leads to EUV emission, and the effects of a flare on the chromosphere are observable in optical and millimeter wavelengths. Thus, joint observation campaigns with X-ray telescopes like NASA's RHESSI, the Japanese Hinode, or the upcoming Solar Dynamics Observatory of NASA, are as reasonable as with optical telescopes like GREGOR or millimeter wave telescopes like ALMA.
  For CME studies, joint observations with LOFAR and space-based radio receivers are useful, since the frequency of CME radio emission decreases with increasing distance from the Sun, and ground-based observations of frequencies below 10 MHz are not possible due to ionospheric reflection. An example for such a space mission is STEREO.
• Single station as a spectrometer
  Beside of the previously presented observation modes that make use of the whole LOFAR system for image synthesis, there is also the possibility to use only a single station for solar observations. This could be possible if e.g. a LOFAR observing program only uses the central core, and remote stations like that at Bornim are available. In this case, the radio spectrum of the Sun is observed in selected frequency intervals starting from 30 MHz. Such a program makes optimal use of the resources of the LOFAR telescope.

More informationen on solar radio astronomy with LOFAR is available in our brochure Solar Physics with LOFAR (PDF, 1071 KB).

• Solar-stellar connections
  The Sun is the only star that can be observed directly from Earth with imaging telescopes. Investigation of the Sun as a typical main sequence star is thus of great interest for stellar astrophysics.
• Extragalactic astronomy
  LOFAR is capable of observing the formation and evolution of galaxies, galaxy clusters, and active galactic nuclei (AGNs), as well as the epoch of reionization of matter in the early Universe due to the UV light of the first stars and quasars.
• Galactic astronomy
  With LOFAR it will be possible to study the absorption and polarization of radio waves in the interstellar medium, as well as shocks and particle acceleration at supernova remnants.
• All sky surveys
  LOFAR will produce radio maps of the whole sky visible from Central Europe with unprecedented sensitivity and angular resolution. The discovery of many new objects can be expected.
• GRID - computing
  LOFAR observations produce large amounts of data. Their processing and storage requires a suitable infrastructure that is provided by GRID technology, e.g. dCache. Thus, GRID computing and E-science are essential components of the LOFAR project.