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Pulsars exhibit variations or scintillation of radio emission in time and frequency caused by electron density inhomogeneities of the interstellar medium. Two sources with transverse separation more than the size of the diffracton pattern at the Earth will produce independent scintillations from these sources. We used this idea to resolve pulsar magnetospheres. Here we present new observations of 3 pulsars --- PSR 0834+6, 1133+16, and 1919+21 --- in which we can see evident decorrelation of the pulsar spectrum with increasing space separation between sources in the pulsar magnetosphere.
Measurements were made at 102.7 MHz using the Large-Aperture Synthesis Telescope (BSA) of the Lebedev Physics Institute at Pushchino (Russia) in April and June, 1994. We used a new $128\times 1.25$ KHz multichannel receiver. Pulses were sampled every 5.12 ms in all 128 channels in a 300 ms window. We used the noise part of each recording for a baseline calibration and equalization of amplification in all channels.
For studing the decorrelation between pulsar spectra at separated longitudes we have calculated a mean cross-correlation functions between strong pulse ( $S/N \geq 5\sigma $) spectra taken at the longitude of the leading edge of the pulse profile and at all subsequent longitudes $l$. We will present the dynamic spectra of PSR 0834+06, 1133+16 and 1919+21 at separated longitudes. We can see obvious decorrelation of the spectra at remote longitudes. The strongest difference between spectra occur for PSR 1919+21. From the falling crosscorrelation coefficient $R(0)$ with increasing longitude separation $\Delta l$ we can obtain the space separation $\Delta r$ between sources.
