## 4 Discussion  
The meaningful influence of the ionosphere on LOFAR observations was discussed during the development phase of this instrument, when it was pointed out that it could be an independent source of knowledge about this layer of the atmosphere [5]. Studies of the influence of the ionosphere on the observations of astronomical sources have been conducted since the very beginning of the telescope, but they are mainly focused on space weather [7; 8; 9]. The recent results show the scrutiny of the ionospheric scintillations of the radio signals from the two distant radio sources [3] and present the sensitivity of the LOFAR instrument. We also present an ionospheric scintillation spectrum in the Figure 6 as an example of the LOFAR sensitivity for ionosphere density irregularities. Pulsar surveys using the LOFAR telescope are conducted either in a special mode, where single station is used for observation [35], as well as the central area (i.e., the LOFAR Core) is used in the mode of creating multibeam and data analysis (see e.g., [34; 59; 60]). For the majority of pulsar observations conducted with LOFAR the influence of the ionosphere is neglected and treated only as a nuisance, decreasing the sensitivity of the instrument. A large portion of pulsar observing projects does not require absolute flux density measurements (which in fact is not an easy task for LOFAR) and in such cases the estimation of relative flux density variations is more than sufficient [61]. There are also investigations which are using ILT imaging mode [62]. Only the last of the mentioned observation modes during data correlation reduces the averaged influence of the ionosphere. As we mentioned before, the need for calibration for disturbances in the wave front caused by the ionosphere was taken into account and discussed already in the instrument's construction phase [63; 64], particularly in imaging interferometric mode. However, data obtained in beamforming mode (single station and core) are subject to minimal analysis of  
Figure 11: The plot at the bottom part shows fluctuations of relative flux for the entire period of observation of the pulsar PSR J0332+5434 on May 20, 2017. The level of relative flux was normalized. The green lines show close-ups time (see Figures 6 and 9). The height of the pulsar above the horizon-H and the signal-to-noise ratio (S/N) are also noted. Top panel presents the latitudinal extent of the Rate of TEC Index (ROTI) auroral oval during one day. the influence of the ionosphere, in most cases, the results obtained after a long integration time (10-15 min and longer) are taken for analysis, which averages the results for distorting the direction of view, and thus shifts the target to the edge of the beam or outside of it, which ultimately causes changes in the recorded value of relative flux of the pulsar profile. In Figure 12, in a suggestive way, we show what the danger may be when analyzing the data if we do not take into account the ionosphere influence. While the average profile blurs the influence of signal changes along the way through the ionosphere, in single profiles this influence may be significant. We may come to the point when we are not able to definitely decide whether the changes from pulse to pulse are related to the dynamics of the ionosphere or to the source itself, which is a neutron star. There are, however, a few pulsar observing techniques, where the influence of the ionospheric variations can be very impactful. The most important of these is the single-pulse observing mode, where the incoming pulsar signal is not integrated and a full time series comprising of individual pulses is analyzed. Unfortunately, the analysis of the influence of the ionosphere is not taken into account during the analysis of such observations (see e.g., [65; 66]). This is due to the fact that there is usually no external information on the state of the ionosphere during the observation and in the direction of the source.